diff --git a/cmd/felis/api.go b/cmd/felis/api.go
index d0343a6..f281b18 100644
--- a/cmd/felis/api.go
+++ b/cmd/felis/api.go
@@ -300,8 +300,20 @@ func cmdAPI(args []string, stdout, stderr io.Writer) int {
 		// for legitimate multi-tab / multi-server watching, while capping how many
 		// upstream follow connections a single caller can tie up if their streams stall.
 		MaxStreamsPerPrincipal: 16,
+		// Public sign-in doors, per client address: a person signing in makes a
+		// handful of calls, so 20 at once refilled at 20 a minute never bites a
+		// real user and still turns a spray into a trickle. The client address
+		// is the edge's header when the install names one (config.AuthConfig).
+		AuthDoorLimit:  api.RateLimit{Burst: 20, PerMinute: 20},
+		ClientIPHeader: cfg.Auth.EffectiveClientIPHeader(),
+		MailLimit:      mailLimit(cfg.SMTP.MaxPerHour),
 	}
 	fmt.Fprintln(stderr, "felis api: external face fails closed (Access JWKS key function not configured)")
+	if a.ClientIPHeader != "" {
+		fmt.Fprintf(stderr, "felis api: sign-in rate limit keys on the %s header\n", a.ClientIPHeader)
+	} else {
+		fmt.Fprintln(stderr, "felis api: sign-in rate limit keys on the TCP peer ([auth] client_ip_header unset)")
+	}
 
 	// Felis-nano: the multi-source hasJoined multiplexer. Mojang leads as the code-owned
 	// identity anchor (正版优先); config can only append namespace-rewritten third-party
@@ -546,3 +558,13 @@ func reconcileBuilds(ctx context.Context, b *build.Builder, stderr io.Writer) {
 		}
 	}
 }
+
+// mailLimit turns smtp.max_per_hour into the API's install-wide mail bucket:
+// the hourly cap as the refill rate, with a quarter of it (at least 5) allowed
+// at once so a burst of real sign-ins is not queued behind the average.
+func mailLimit(perHour int) api.RateLimit {
+	if perHour <= 0 {
+		perHour = config.DefaultMailPerHour
+	}
+	return api.RateLimit{Burst: max(perHour/4, 5), PerMinute: float64(perHour) / 60}
+}
diff --git a/cmd/felis/tui_edge_apply.go b/cmd/felis/tui_edge_apply.go
index 107b304..cc8e68a 100644
--- a/cmd/felis/tui_edge_apply.go
+++ b/cmd/felis/tui_edge_apply.go
@@ -32,7 +32,9 @@ func applyCloudflareEdge(ctx context.Context, result *cfsetup.Result, panelHost,
 	if adminHost == "" {
 		return fmt.Errorf("admin hostname is required")
 	}
-	if err := writeConnectionConfig(panelHost, adminHost, result.AccessAud); err != nil {
+	// cloudflared is the only way in once the NodePort is fenced, so the
+	// visitor address it writes can key the sign-in rate limit.
+	if err := writeConnectionConfig(panelHost, adminHost, result.AccessAud, "CF-Connecting-IP"); err != nil {
 		return err
 	}
 	if err := applyFelisConfigSecret(ctx); err != nil {
@@ -78,7 +80,8 @@ func applyReverseProxy(ctx context.Context, panelHost, adminHost string) error {
 	if adminHost == "" {
 		return fmt.Errorf("admin hostname is required")
 	}
-	if err := writeConnectionConfig(panelHost, adminHost, ""); err != nil {
+	// Caddy, nginx and Traefik all append the peer they saw to X-Forwarded-For.
+	if err := writeConnectionConfig(panelHost, adminHost, "", "X-Forwarded-For"); err != nil {
 		return err
 	}
 	if err := applyFelisConfigSecret(ctx); err != nil {
@@ -93,16 +96,17 @@ func applyReverseProxy(ctx context.Context, panelHost, adminHost string) error {
 // writeConnectionConfig stamps the chosen hostnames (and optional Access audience)
 // into both the host and pod config files. An empty aud clears any prior
 // Cloudflare audience, which is correct when switching to a non-Access front.
-func writeConnectionConfig(panelHost, adminHost, aud string) error {
+// clientIPHeader is the header that front writes the visitor address into.
+func writeConnectionConfig(panelHost, adminHost, aud, clientIPHeader string) error {
 	for _, path := range []string{hostSetupConfigPath, podSetupConfigPath} {
-		if err := updateAuthConfig(path, panelHost, adminHost, aud); err != nil {
+		if err := updateAuthConfig(path, panelHost, adminHost, aud, clientIPHeader); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func updateAuthConfig(path, panelHost, adminHost, aud string) error {
+func updateAuthConfig(path, panelHost, adminHost, aud, clientIPHeader string) error {
 	cfg, err := config.Load(path)
 	if err != nil {
 		return err
@@ -112,6 +116,7 @@ func updateAuthConfig(path, panelHost, adminHost, aud string) error {
 	}
 	cfg.Auth.AdminHostname = adminHost
 	cfg.Auth.AccessJWTAud = aud
+	cfg.Auth.ClientIPHeader = clientIPHeader
 	return writeConfig(path, cfg)
 }
 
diff --git a/deploy/alerts/felis-alerts.yaml b/deploy/alerts/felis-alerts.yaml
index f4fc9c0..929ae9f 100644
--- a/deploy/alerts/felis-alerts.yaml
+++ b/deploy/alerts/felis-alerts.yaml
@@ -4,7 +4,9 @@
 #
 # felis_* series come from two processes:
 #   - felis-operator pod :8080/metrics      → felis_servers_total, felis_start_duration_seconds
-#   - felis-api internal :8081/metrics      → felis_image_build_failures_total
+#   - felis-api internal :8081/metrics      → felis_image_build_failures_total,
+#                                             felis_mail_total, felis_rate_limited_total,
+#                                             felis_auth_otp_lockouts_total
 #   - node-exporter textfile collector      → felis_db_backup_* (felis-db-backup.timer)
 # node_* / kube_* series come from node-exporter / kube-state-metrics.
 groups:
@@ -94,3 +96,49 @@ groups:
             --collector.textfile.directory at the directory of
             FELIS_DB_BACKUP_METRICS (default /var/lib/node_exporter/textfile_collector)
             (troubleshooting §16).
+  - name: felis.auth.rules
+    rules:
+      - alert: FelisMailBudgetExhausted
+        expr: sum(increase(felis_mail_total{result="throttled"}[15m])) > 0
+        labels:
+          severity: warning
+        annotations:
+          summary: "the install-wide mail budget refused mail"
+          description: >-
+            felis_mail_total{result="throttled"} increased: [smtp] max_per_hour is
+            spent, and every sign-in code is refused with 429 mail_rate_limited until
+            it refills. Check felis_rate_limited_total for a flood before raising the
+            budget (troubleshooting §17).
+      - alert: FelisMailDeliveryFailing
+        expr: sum(increase(felis_mail_total{result="failed"}[15m])) > 0
+        labels:
+          severity: warning
+        annotations:
+          summary: "the SMTP relay refused mail in the last 15m"
+          description: >-
+            felis_mail_total{result="failed"} increased: sign-in codes are not being
+            delivered (502 mail_undeliverable). The relay's reason is in the
+            felis-api log (troubleshooting §17).
+      - alert: FelisSignInFlood
+        expr: sum(rate(felis_rate_limited_total{scope="auth_door"}[5m])) * 60 > 10
+        for: 10m
+        labels:
+          severity: warning
+        annotations:
+          summary: "sign-in doors refusing over 10 requests a minute"
+          description: >-
+            The per-address sign-in limit has been refusing callers for 10 minutes.
+            A script is hammering the auth doors; if real users report rate_limited
+            at once instead, [auth] client_ip_header is missing and everyone shares
+            the proxy's address (troubleshooting §17).
+      - alert: FelisOTPAccountLocked
+        expr: sum by (purpose) (increase(felis_auth_otp_lockouts_total[1h])) > 0
+        labels:
+          severity: warning
+        annotations:
+          summary: "an account's email-code sign-in locked after 10 wrong codes"
+          description: >-
+            Someone entered 10 wrong codes for one account within 24h ({{ $labels.purpose }}).
+            The audit log names the account (action auth.otp.locked); the owner was
+            mailed. Unless they fumbled codes, someone is guessing at it
+            (troubleshooting §17).
diff --git a/deploy/alerts/felis-alerts_test.yml b/deploy/alerts/felis-alerts_test.yml
index 24071ef..322ae22 100644
--- a/deploy/alerts/felis-alerts_test.yml
+++ b/deploy/alerts/felis-alerts_test.yml
@@ -151,3 +151,88 @@ tests:
                 --collector.textfile.directory at the directory of
                 FELIS_DB_BACKUP_METRICS (default /var/lib/node_exporter/textfile_collector)
                 (troubleshooting §16).
+  - name: sign-in mail budget and relay
+    interval: 1m
+    input_series:
+      # Created at zero on start; the budget refuses one mail at t=3m.
+      - series: 'felis_mail_total{kind="otp",result="throttled",job="felis-api"}'
+        values: '0 0 0 1x30'
+      - series: 'felis_mail_total{kind="otp",result="failed",job="felis-api"}'
+        values: '0x33'
+    alert_rule_test:
+      - eval_time: 2m
+        alertname: FelisMailBudgetExhausted
+        exp_alerts: []
+      - eval_time: 5m
+        alertname: FelisMailBudgetExhausted
+        exp_alerts:
+          - exp_labels:
+              severity: warning
+            exp_annotations:
+              summary: "the install-wide mail budget refused mail"
+              description: >-
+                felis_mail_total{result="throttled"} increased: [smtp] max_per_hour is
+                spent, and every sign-in code is refused with 429 mail_rate_limited until
+                it refills. Check felis_rate_limited_total for a flood before raising the
+                budget (troubleshooting §17).
+      - eval_time: 5m
+        alertname: FelisMailDeliveryFailing
+        exp_alerts: []
+  - name: sign-in flood
+    interval: 1m
+    input_series:
+      # 30 refusals a minute from t=0; a lone refused script at 2/min stays quiet.
+      - series: 'felis_rate_limited_total{scope="auth_door",job="felis-api"}'
+        values: '0+30x40'
+    alert_rule_test:
+      - eval_time: 10m
+        alertname: FelisSignInFlood
+        exp_alerts: []
+      - eval_time: 20m
+        alertname: FelisSignInFlood
+        exp_alerts:
+          - exp_labels:
+              severity: warning
+            exp_annotations:
+              summary: "sign-in doors refusing over 10 requests a minute"
+              description: >-
+                The per-address sign-in limit has been refusing callers for 10 minutes.
+                A script is hammering the auth doors; if real users report rate_limited
+                at once instead, [auth] client_ip_header is missing and everyone shares
+                the proxy's address (troubleshooting §17).
+  - name: sign-in trickle stays quiet
+    interval: 1m
+    input_series:
+      - series: 'felis_rate_limited_total{scope="auth_door",job="felis-api"}'
+        values: '0+2x40'
+    alert_rule_test:
+      - eval_time: 30m
+        alertname: FelisSignInFlood
+        exp_alerts: []
+  - name: account email-code lock
+    interval: 1m
+    input_series:
+      - series: 'felis_auth_otp_lockouts_total{purpose="login_email",job="felis-api"}'
+        values: '0 0 1x90'
+      - series: 'felis_auth_otp_lockouts_total{purpose="op_login",job="felis-api"}'
+        values: '0x92'
+    alert_rule_test:
+      - eval_time: 1m
+        alertname: FelisOTPAccountLocked
+        exp_alerts: []
+      - eval_time: 10m
+        alertname: FelisOTPAccountLocked
+        exp_alerts:
+          - exp_labels:
+              severity: warning
+              purpose: login_email
+            exp_annotations:
+              summary: "an account's email-code sign-in locked after 10 wrong codes"
+              description: >-
+                Someone entered 10 wrong codes for one account within 24h (login_email).
+                The audit log names the account (action auth.otp.locked); the owner was
+                mailed. Unless they fumbled codes, someone is guessing at it
+                (troubleshooting §17).
+      - eval_time: 90m
+        alertname: FelisOTPAccountLocked
+        exp_alerts: []
diff --git a/deploy/alerts/felis-prometheusrule.yaml b/deploy/alerts/felis-prometheusrule.yaml
index 01cb6bb..0b68db9 100644
--- a/deploy/alerts/felis-prometheusrule.yaml
+++ b/deploy/alerts/felis-prometheusrule.yaml
@@ -98,3 +98,49 @@ spec:
               --collector.textfile.directory at the directory of
               FELIS_DB_BACKUP_METRICS (default /var/lib/node_exporter/textfile_collector)
               (troubleshooting §16).
+    - name: felis.auth.rules
+      rules:
+        - alert: FelisMailBudgetExhausted
+          expr: sum(increase(felis_mail_total{result="throttled"}[15m])) > 0
+          labels:
+            severity: warning
+          annotations:
+            summary: "the install-wide mail budget refused mail"
+            description: >-
+              felis_mail_total{result="throttled"} increased: [smtp] max_per_hour is
+              spent, and every sign-in code is refused with 429 mail_rate_limited until
+              it refills. Check felis_rate_limited_total for a flood before raising the
+              budget (troubleshooting §17).
+        - alert: FelisMailDeliveryFailing
+          expr: sum(increase(felis_mail_total{result="failed"}[15m])) > 0
+          labels:
+            severity: warning
+          annotations:
+            summary: "the SMTP relay refused mail in the last 15m"
+            description: >-
+              felis_mail_total{result="failed"} increased: sign-in codes are not being
+              delivered (502 mail_undeliverable). The relay's reason is in the
+              felis-api log (troubleshooting §17).
+        - alert: FelisSignInFlood
+          expr: sum(rate(felis_rate_limited_total{scope="auth_door"}[5m])) * 60 > 10
+          for: 10m
+          labels:
+            severity: warning
+          annotations:
+            summary: "sign-in doors refusing over 10 requests a minute"
+            description: >-
+              The per-address sign-in limit has been refusing callers for 10 minutes.
+              A script is hammering the auth doors; if real users report rate_limited
+              at once instead, [auth] client_ip_header is missing and everyone shares
+              the proxy's address (troubleshooting §17).
+        - alert: FelisOTPAccountLocked
+          expr: sum by (purpose) (increase(felis_auth_otp_lockouts_total[1h])) > 0
+          labels:
+            severity: warning
+          annotations:
+            summary: "an account's email-code sign-in locked after 10 wrong codes"
+            description: >-
+              Someone entered 10 wrong codes for one account within 24h ({{ $labels.purpose }}).
+              The audit log names the account (action auth.otp.locked); the owner was
+              mailed. Unless they fumbled codes, someone is guessing at it
+              (troubleshooting §17).
diff --git a/docs/openapi.yaml b/docs/openapi.yaml
index f53d27d..13afb3d 100644
--- a/docs/openapi.yaml
+++ b/docs/openapi.yaml
@@ -147,6 +147,19 @@ components:
       content:
         application/json:
           schema: { $ref: '#/components/schemas/Error' }
+    RateLimited:
+      description: >
+        This client address called the public sign-in doors faster than the per-address
+        limit allows (code rate_limited); Retry-After gives the seconds until the next
+        call is admitted. The address is the visitor header the install's edge writes
+        ([auth] client_ip_header: CF-Connecting-IP behind the Cloudflare tunnel), else
+        the TCP peer; IPv6 clients share one limit per /64.
+      headers:
+        Retry-After:
+          schema: { type: integer }
+      content:
+        application/json:
+          schema: { $ref: '#/components/schemas/Error' }
     AccessResult:
       description: The structured access mutation succeeded; the raw RCON reply is in output.
       content:
@@ -1875,8 +1888,8 @@ paths:
         array. It never reveals staffness: methods are computed identically for every
         resolved account (no role branch), so a staff and a player address in the same
         credential state return byte-identical bodies. passkey is offered only when a
-        verifier is wired. Sends no mail and mutates nothing; not rate-limited at the app
-        layer (volumetric abuse is bounded at the edge). Gated on local_auth_enabled.
+        verifier is wired. Sends no mail and mutates nothing; bounded by the per-address
+        sign-in rate limit (429 rate_limited). Gated on local_auth_enabled.
       x-felis-face: [external]
       x-felis-tier: public
       security: []
@@ -1918,6 +1931,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
 
   /api/v1/auth/passkey/login/begin:
     post:
@@ -1973,7 +1988,9 @@ paths:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
         '429':
-          description: A passkey login for this recipient was started too recently (otp_resend_cooldown).
+          description: >-
+            A passkey login for this recipient was started too recently (otp_resend_cooldown);
+            or this client address called the sign-in doors too often (rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
@@ -2045,6 +2062,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
         '503':
           description: No passkey verifier is wired on this deployment (passkey_unavailable).
           content:
@@ -2066,9 +2085,9 @@ paths:
         userHandle inside the signed assertion at finish. The challenge cannot be
         user-keyed, so it is stashed under login_id in a non-user-keyed store and echoed
         back at finish. Mounted Public and gated on local_auth_enabled. There is no
-        recipient or principal to key a per-caller cooldown on (that volumetric limiting
-        is delegated to the edge), so the server-side brake is a hard global cap on live
-        challenges (429 too_many_challenges). Inert for a credential until its owner
+        recipient or principal to key a per-caller cooldown on, so one client is bounded
+        by the per-address sign-in rate limit (429 rate_limited) and the table by a hard
+        global cap on live challenges (429 too_many_challenges). Inert for a credential until its owner
         enrolls a resident passkey; email-OTP and username-first passkey remain the
         fallbacks, so no authenticator is ever locked out.
       x-felis-face: [external]
@@ -2114,8 +2133,9 @@ paths:
               schema: { $ref: '#/components/schemas/Error' }
         '429':
           description: >-
-            Too many discoverable logins are in flight server-wide; the global cap is hit
-            (too_many_challenges). No per-recipient signal is leaked — the cap is global.
+            Too many discoverable logins are in flight server-wide (too_many_challenges;
+            the cap is global, so no per-recipient signal leaks); or this client address
+            called the sign-in doors too often (rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
@@ -2192,6 +2212,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
         '503':
           description: No passkey verifier is wired on this deployment (passkey_unavailable).
           content:
@@ -2253,7 +2275,10 @@ paths:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
         '429':
-          description: A code for this recipient was requested too recently (otp_resend_cooldown).
+          description: >-
+            A code for this recipient was requested too recently (otp_resend_cooldown);
+            or this client address called the sign-in doors too often (rate_limited, with Retry-After);
+            or the install-wide mail budget is spent (mail_rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
@@ -2318,6 +2343,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
 
   /api/v1/auth/op-login/start:
     post:
@@ -2373,7 +2400,10 @@ paths:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
         '429':
-          description: A code for this recipient was requested too recently (otp_resend_cooldown).
+          description: >-
+            A code for this recipient was requested too recently (otp_resend_cooldown);
+            or this client address called the sign-in doors too often (rate_limited, with Retry-After);
+            or the install-wide mail budget is spent (mail_rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
@@ -2467,6 +2497,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
 
   /api/v1/auth/setup/redeem:
     post:
@@ -2524,6 +2556,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
 
   /api/v1/auth/setup/status:
     get:
@@ -2640,6 +2674,8 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
+        '429':
+          $ref: '#/components/responses/RateLimited'
 
   /api/v1/me:
     get:
@@ -3778,7 +3814,8 @@ paths:
           description: >-
             Resend requested before the cooldown elapsed (otp_resend_cooldown); or the
             account spent its daily wrong-code budget (otp_account_locked, with
-            Retry-After).
+            Retry-After); or the install-wide mail budget is spent
+            (mail_rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
@@ -4091,7 +4128,8 @@ paths:
           description: >-
             Resend requested before the cooldown elapsed (otp_resend_cooldown), or the
             account's daily wrong-code budget is spent (otp_account_locked, with
-            Retry-After).
+            Retry-After); or the install-wide mail budget is spent
+            (mail_rate_limited, with Retry-After).
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Error' }
diff --git a/docs/troubleshooting.md b/docs/troubleshooting.md
index 5e4f56d..538cdcc 100644
--- a/docs/troubleshooting.md
+++ b/docs/troubleshooting.md
@@ -931,7 +931,9 @@ The series come from two processes:
   `felis_start_duration_seconds` (no Service; scrape pod-scoped, e.g. a
   PodMonitor targeting port `metrics`).
 - `felis-api` internal face `:8081/metrics` (Service `felis-api-internal`) —
-  `felis_image_build_failures_total`. Unauthenticated like the probes;
+  `felis_image_build_failures_total`, and the sign-in series of §17
+  (`felis_mail_total`, `felis_rate_limited_total`,
+  `felis_auth_otp_lockouts_total`). Unauthenticated like the probes;
   ClusterIP-only, and the external face never serves it.
 - `felis_reaper_worlds_deleted_total` is produced inside the one-shot reaper
   CronJob, which exits long before any scrape interval — without a pushgateway
@@ -941,8 +943,10 @@ The series come from two processes:
 ### Alert rules
 
 `deploy/alerts/` ships ready-made rules: build failures, slow starts, node
-disk/memory thresholds, the kubelet `DiskPressure` condition, and control-plane
-database backup freshness (§16; needs node-exporter's textfile collector).
+disk/memory thresholds, the kubelet `DiskPressure` condition, control-plane
+database backup freshness (§16; needs node-exporter's textfile collector), and
+sign-in abuse: the mail budget, relay failures, throttled floods and account
+code locks (§17).
 
 - Plain Prometheus: add `felis-alerts.yaml` to `rule_files`. Check and unit-test
   it standalone with `promtool check rules felis-alerts.yaml` and
@@ -1167,6 +1171,75 @@ Skips the pre-migration snapshot (`migrate up -no-backup`). The installer warns
 loudly when it is set. Use it only when the snapshot cannot work and you have
 another backup, e.g. an external database newer than the host's `pg_dump`.
 
+## 17. Sign-in refused with 429, mail budget, account code locks
+
+The public sign-in doors (`/api/v1/auth/*` except logout and the op-login
+status poll) have three limits of their own. Each answers 429 with a
+`Retry-After` header and a distinct error code.
+
+### `rate_limited`: one address called the doors too often
+
+Each client address gets 20 calls at once, refilled at 20 a minute, shared
+across every door. A person signing in makes three or four calls, so this only
+bites scripts. IPv6 clients share one limit per /64. Refusals count in
+`felis_rate_limited_total{scope="auth_door"}`; `FelisSignInFlood` fires when
+more than 10 a minute are refused for 10 minutes.
+
+The address comes from `[auth] client_ip_header`:
+
+- Behind the Cloudflare tunnel it is `CF-Connecting-IP`. The edge setup writes
+  it, and an install with an `access_jwt_aud` implies it. The header is
+  trustworthy there because the same setup fences the panel NodePort to
+  loopback, so every request reaching the API came through cloudflared.
+- Behind your own reverse proxy it is `X-Forwarded-For` (the rightmost entry,
+  the one your proxy appended). Firewall the NodePort so only the proxy reaches
+  it, or a direct caller can write any address it likes.
+- Unset, the TCP peer is used. Behind any proxy every visitor then shares the
+  proxy's address and one limit, so **everyone gets `rate_limited` at once**.
+  The `felis api` log says at start which it keys on (`sign-in rate limit keys
+  on ...`). Set the header in `/etc/felis/felis.toml` and
+  `/etc/felis/felis.pod.toml`, then `felis converge`.
+
+### `mail_rate_limited`: the install-wide mail budget is spent
+
+Every code and notice the API mails spends one token of a single budget,
+`[smtp] max_per_hour` (default 120; a quarter of it may go at once), so a flood
+cannot burn the relay's quota and get the sending account suspended. While it
+is spent, every address gets the same 429 and nothing reaches the relay.
+`felis_mail_total{result="throttled"}` counts refusals and
+`FelisMailBudgetExhausted` fires on the first one. Look at
+`felis_rate_limited_total` first: a flood shows there. If sign-ins are real,
+raise `max_per_hour` to what your relay allows.
+
+`FelisMailDeliveryFailing` is the other half: the relay itself refused mail
+(`felis_mail_total{result="failed"}`, 502 `mail_undeliverable` to the caller).
+The relay's reason is in the `felis-api` log.
+
+### `otp_account_locked`: ten wrong codes in 24 hours
+
+Ten wrong email codes for one account within 24 hours, counted across every
+code it was sent, lock that account's email-code sign-in until 24 hours after
+the first miss. The public doors answer a locked account exactly like a wrong
+code, and the owner gets one mail saying so. Signed-in doors (email
+verification, migration step-up) answer 429 `otp_account_locked`. Passkey
+sign-in keeps working. Each lock is audited as `auth.otp.locked` and counted in
+`felis_auth_otp_lockouts_total{purpose}` (`FelisOTPAccountLocked`).
+
+To lift a lock early once you have confirmed the owner locked themselves out:
+
+```sh
+sudo -u postgres psql felis -c \
+  "DELETE FROM otp_failure_windows WHERE user_id = (SELECT id FROM users WHERE username = '<name>');"
+```
+
+### Optional: a Cloudflare rate limiting rule in front
+
+The limits above live in the API, so they hold on any edge. Behind Cloudflare
+you can also stop floods before they reach the tunnel: Security → WAF → Rate
+limiting rules, match URI Path starts with `/api/v1/auth/` on the console and
+op.console hostnames, count by IP, 30 requests per 10 seconds, action Block
+for 10 seconds (the Free plan's limits).
+
 ---
 
 ## Quick reference: symptom → section
@@ -1198,3 +1271,6 @@ another backup, e.g. an external database newer than the host's `pg_dump`.
 | `pre-migration backup failed, nothing applied` during an upgrade | §16 |
 | Undo a mistaken change / restore the control-plane database | §16 |
 | Host lost: rebuild from a database bundle | §16 |
+| Sign-in 429 `rate_limited` for everyone at once | §17 |
+| 429 `mail_rate_limited` / `FelisMailBudgetExhausted` | §17 |
+| Right code refused; `otp_account_locked` / `FelisOTPAccountLocked` | §17 |
diff --git a/internal/api/api.go b/internal/api/api.go
index 902a319..6699e7c 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -158,6 +158,16 @@ type API struct {
 	// Consumed by handleHasJoined (handlers_hasjoined.go).
 	AuthSources []AuthSource
 
+	// AuthDoorLimit bounds how often one client address may call the public
+	// pre-session auth doors (ratelimit.go). MailLimit bounds all mail the API
+	// sends, install-wide. Zero values disable them; cmd/felis wires both.
+	AuthDoorLimit RateLimit
+	MailLimit     RateLimit
+	// ClientIPHeader names the header the install's edge writes the client
+	// address into (CF-Connecting-IP behind the Cloudflare tunnel,
+	// X-Forwarded-For behind an operator proxy). Empty means the TCP peer.
+	ClientIPHeader string
+
 	// Now is the clock, injectable for tests. Defaults to time.Now.
 	Now func() time.Time
 
@@ -172,6 +182,11 @@ type API struct {
 
 	streamCapOnce sync.Once
 	streamCap     *streamLimiter
+
+	authDoorOnce    sync.Once
+	authDoorBuckets *bucketSet
+	mailOnce        sync.Once
+	mailBuckets     *bucketSet
 }
 
 // panelURL returns the public player-console origin ("https://console.<root>"),
@@ -286,6 +301,11 @@ type apiRoute struct {
 	// whose EmailVerified is false is restricted to these routes only.
 	SetupAllowed bool
 
+	// AuthDoor marks a public pre-session auth door: it is rate limited per
+	// client address (throttleAuthDoor). The op-login status poll is left off,
+	// since the browser calls it every few seconds while it waits.
+	AuthDoor bool
+
 	h http.HandlerFunc
 }
 
@@ -381,21 +401,21 @@ func (a *API) externalAPIRoutes() []apiRoute {
 		// counter-slice to the anti-enumeration doors — the ONE sanctioned place existence
 		// is disclosed — but it never reveals staffness (methods computed with no role
 		// branch, so a staff and a player address in the same state are indistinguishable).
-		{Method: "POST", Pattern: "/api/v1/auth/options", Public: true, h: a.handleAuthOptions},
-		{Method: "POST", Pattern: "/api/v1/auth/setup/redeem", Public: true, h: a.handleSetupRedeem},
+		{Method: "POST", Pattern: "/api/v1/auth/options", Public: true, AuthDoor: true, h: a.handleAuthOptions},
+		{Method: "POST", Pattern: "/api/v1/auth/setup/redeem", Public: true, AuthDoor: true, h: a.handleSetupRedeem},
 		{Method: "GET", Pattern: "/api/v1/auth/setup/status", SetupAllowed: true, h: a.handleSetupStatus},
-		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/begin", Public: true, h: a.handlePasskeyLoginBegin},
-		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/finish", Public: true, h: a.handlePasskeyLoginFinish},
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/begin", Public: true, AuthDoor: true, h: a.handlePasskeyLoginBegin},
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/finish", Public: true, AuthDoor: true, h: a.handlePasskeyLoginFinish},
 		// Discoverable ("usernameless") passkey login (task #40): the from-zero sibling of the
 		// email-first pair above — no identifier typed, the account is resolved from the
 		// userHandle inside the signed assertion (handlers_passkey_discoverable.go).
-		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/begin", Public: true, h: a.handlePasskeyLoginDiscoverableBegin},
-		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/finish", Public: true, h: a.handlePasskeyLoginDiscoverableFinish},
-		{Method: "POST", Pattern: "/api/v1/auth/email/start", Public: true, h: a.handleLoginEmailStart},
-		{Method: "POST", Pattern: "/api/v1/auth/email/verify", Public: true, h: a.handleLoginEmailVerify},
-		{Method: "POST", Pattern: "/api/v1/auth/op-login/start", Public: true, h: a.handleOpLoginStart},
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/begin", Public: true, AuthDoor: true, h: a.handlePasskeyLoginDiscoverableBegin},
+		{Method: "POST", Pattern: "/api/v1/auth/passkey/login/discoverable/finish", Public: true, AuthDoor: true, h: a.handlePasskeyLoginDiscoverableFinish},
+		{Method: "POST", Pattern: "/api/v1/auth/email/start", Public: true, AuthDoor: true, h: a.handleLoginEmailStart},
+		{Method: "POST", Pattern: "/api/v1/auth/email/verify", Public: true, AuthDoor: true, h: a.handleLoginEmailVerify},
+		{Method: "POST", Pattern: "/api/v1/auth/op-login/start", Public: true, AuthDoor: true, h: a.handleOpLoginStart},
 		{Method: "GET", Pattern: "/api/v1/auth/op-login/status/{id}", Public: true, h: a.handleOpLoginStatus},
-		{Method: "POST", Pattern: "/api/v1/auth/op-login/finish", Public: true, h: a.handleOpLoginFinish},
+		{Method: "POST", Pattern: "/api/v1/auth/op-login/finish", Public: true, AuthDoor: true, h: a.handleOpLoginFinish},
 		// Player-console bootstrap (console-tier access model): the account-less
 		// player's door into console.<root_domain>. Public — like login there is no prior
 		// principal — and session-minting, but the artifact it consumes is a one-time
@@ -403,7 +423,7 @@ func (a *API) externalAPIRoutes() []apiRoute {
 		// possession already proves a Minecraft identity. A code whose UUID belongs to
 		// staff is refused (403) so this never yields an admin session; op.console stays
 		// behind Zero Trust (handlers_onboard.go).
-		{Method: "POST", Pattern: "/api/v1/auth/bind", Public: true, h: a.handleBindRedeem},
+		{Method: "POST", Pattern: "/api/v1/auth/bind", Public: true, AuthDoor: true, h: a.handleBindRedeem},
 
 		// App-auth tier: operations on your own servers (spec §14).
 		{Method: "POST", Pattern: "/api/v1/servers/{name}/wake", h: a.handleWake},
@@ -614,7 +634,11 @@ func (a *API) buildFace(routes []apiRoute, guard func(http.Handler) http.Handler
 	for _, rt := range routes {
 		pattern := rt.Method + " " + rt.Pattern
 		if rt.Public {
-			mux.HandleFunc(pattern, rt.h)
+			h := rt.h
+			if rt.AuthDoor {
+				h = a.throttleAuthDoor(h)
+			}
+			mux.HandleFunc(pattern, h)
 			continue
 		}
 		h := rt.h
@@ -727,10 +751,35 @@ func principalFromContext(ctx context.Context) *Principal {
 // reserve/release pair closes the intra-replica concurrent burst (the bug fixed in
 // #35); cross-replica bounding would need a shared store (out of scope for the
 // single-replica demo).
+//
+// Entries older than the longest window the limiter has been asked about can
+// no longer block anything, so checks sweep them out (at most once per
+// bucketSweepEvery). Without that, every distinct address typed into a public
+// door, whose neutral branch keeps its reservation, stayed in the map for the
+// life of the process.
 type cooldownLimiter struct {
-	mu   sync.Mutex
-	now  func() time.Time
-	last map[string]time.Time
+	mu        sync.Mutex
+	now       func() time.Time
+	last      map[string]time.Time
+	maxWindow time.Duration
+	swept     time.Time
+}
+
+// noteWindow widens the retention to window and sweeps stale entries when due.
+// The caller holds mu.
+func (c *cooldownLimiter) noteWindow(window time.Duration, now time.Time) {
+	if window > c.maxWindow {
+		c.maxWindow = window
+	}
+	if c.maxWindow <= 0 || now.Sub(c.swept) < bucketSweepEvery {
+		return
+	}
+	c.swept = now
+	for k, t := range c.last {
+		if now.Sub(t) >= c.maxWindow {
+			delete(c.last, k)
+		}
+	}
 }
 
 // allowed reports whether name may wake now WITHOUT recording the attempt. A
@@ -745,7 +794,9 @@ func (c *cooldownLimiter) allowed(name string, window time.Duration) bool {
 	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	if last, ok := c.last[name]; ok && c.now().Sub(last) < window {
+	now := c.now()
+	c.noteWindow(window, now)
+	if last, ok := c.last[name]; ok && now.Sub(last) < window {
 		return false
 	}
 	return true
@@ -778,10 +829,11 @@ func (c *cooldownLimiter) reserve(name string, window time.Duration) (time.Time,
 	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	if last, ok := c.last[name]; ok && c.now().Sub(last) < window {
+	t := c.now()
+	c.noteWindow(window, t)
+	if last, ok := c.last[name]; ok && t.Sub(last) < window {
 		return time.Time{}, false
 	}
-	t := c.now()
 	c.last[name] = t
 	return t, true
 }
diff --git a/internal/api/errors.go b/internal/api/errors.go
index 9dea2ed..9bc3f56 100644
--- a/internal/api/errors.go
+++ b/internal/api/errors.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"strconv"
 	"time"
 )
 
@@ -130,10 +131,19 @@ type apiError struct {
 	status int
 	code   string
 	msg    string
+	// wait, when positive, is sent as Retry-After (whole seconds, rounded up).
+	wait time.Duration
 }
 
 func (e *apiError) Error() string { return e.msg }
 
+// retryAfter returns a copy of e that tells the client when to retry.
+func (e *apiError) retryAfter(d time.Duration) *apiError {
+	c := *e
+	c.wait = d
+	return &c
+}
+
 // newError builds an apiError with a formatted message.
 func newError(status int, code, format string, a ...any) *apiError {
 	return &apiError{status: status, code: code, msg: fmt.Sprintf(format, a...)}
@@ -176,6 +186,9 @@ func writeError(w http.ResponseWriter, r *http.Request, err error) {
 			r.Method, r.URL.Path, requestIDFromContext(r.Context()), err)
 		ae = newError(http.StatusInternalServerError, "internal", "internal error")
 	}
+	if ae.wait > 0 {
+		w.Header().Set("Retry-After", strconv.FormatInt(int64((ae.wait+time.Second-1)/time.Second), 10))
+	}
 	body := map[string]any{
 		"error": map[string]string{
 			"code":       ae.code,
diff --git a/internal/api/handlers_auth_email.go b/internal/api/handlers_auth_email.go
index 6e75964..ce1f9e9 100644
--- a/internal/api/handlers_auth_email.go
+++ b/internal/api/handlers_auth_email.go
@@ -24,12 +24,9 @@ import (
 //     this separator).
 //   - No principal. The throttle cannot key off a user id (there is none yet); it
 //     keys off the typed recipient address, the same anti-bomb dimension the onboard
-//     start uses. Per-source (client-IP) aggregate limiting is deliberately NOT done
-//     here: cooldownLimiter is a one-per-window primitive, so keying it on client IP
-//     would false-positive on shared egress (CGNAT / office NAT), and behind
-//     Cloudflare RemoteAddr is the proxy anyway. The only real harm — bombing one
-//     mailbox — is already bounded per recipient; volumetric per-source limiting
-//     belongs at the edge.
+//     start uses. Volume from one client is bounded separately by the per-address
+//     token bucket every public auth door sits behind (throttleAuthDoor), and total
+//     mail by the install-wide mail budget (ratelimit.go).
 //   - Refuse staff. Like handleBindRedeem this public door provably never mints a
 //     session for an admin identity: op.console stays behind Zero Trust (and its own
 //     in-game approval gate). The refusal happens only AFTER a valid code is
@@ -84,6 +81,13 @@ func (a *API) handleLoginEmailStart(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// The install-wide mail budget is checked before the address is resolved,
+	// so while it is spent every address gets the same 429.
+	if err := a.checkMailBudget(); err != nil {
+		writeError(w, r, err)
+		return
+	}
+
 	// Atomically reserve the per-recipient cooldown BEFORE any work, so a burst of
 	// truly concurrent starts yields exactly one winner and each admitted send is one
 	// real, non-idempotent email. The key is namespaced apart from the onboard door's
diff --git a/internal/api/handlers_auth_options.go b/internal/api/handlers_auth_options.go
index dea7853..a96c19d 100644
--- a/internal/api/handlers_auth_options.go
+++ b/internal/api/handlers_auth_options.go
@@ -16,11 +16,10 @@ import (
 // address has an account precisely because THIS endpoint is the one sanctioned place
 // existence is revealed. An empty methods array means "no (verified) account". That
 // makes it a mass-enumeration surface by design — an accepted product decision, the
-// same one the email door's header records. It is bounded only at the edge: the
-// handler sends no mail and mutates nothing, so a per-recipient cooldown would merely
-// block a legitimate retry, and per-source (client-IP) limiting is the edge's job
-// (behind Cloudflare RemoteAddr is the proxy, and CGNAT would false-positive) — see
-// the handlers_auth_email.go header for the same reasoning.
+// same one the email door's header records. The handler sends no mail and mutates
+// nothing, so a per-recipient cooldown would merely block a legitimate retry; what
+// bounds enumeration is the per-client-address token bucket shared by every public
+// auth door (throttleAuthDoor in ratelimit.go).
 //
 // It never reveals STAFFNESS. Methods are computed by the SAME rule for every resolved
 // account — no role branch, no operator hint — so a staff email and a player email in
diff --git a/internal/api/handlers_email_otp.go b/internal/api/handlers_email_otp.go
index a9fc1b6..1b9502c 100644
--- a/internal/api/handlers_email_otp.go
+++ b/internal/api/handlers_email_otp.go
@@ -11,6 +11,8 @@ import (
 	"net/http"
 	"strings"
 	"time"
+
+	"felis.lolicon.best/internal/metrics"
 )
 
 // Player email verification (spec §B2 onboarding). Forced web onboarding proves a
@@ -250,12 +252,21 @@ func (a *API) handleEmailOTPVerify(w http.ResponseWriter, r *http.Request) {
 // deliverOTP hands the code to the configured Mailer, or — when none is wired (the
 // demo) — logs it server-side as a KNOWN-LIMITATION. The code is logged ONLY in the
 // no-mailer fallback and ONLY to the server log; it is never put in an HTTP response.
+//
+// Every real send spends one token of the install-wide mail budget (mailGate);
+// a spent budget is a 429 mail_rate_limited and nothing reaches the relay.
 func (a *API) deliverOTP(ctx context.Context, email, code string) error {
 	if a.Mailer == nil {
 		log.Printf("email-otp: no Mailer configured; code for %s is %s (KNOWN-LIMITATION: demo has no SMTP)", email, code)
 		return nil
 	}
+	if ok, wait := a.mailGate().take(mailGateKey); !ok {
+		metrics.MailTotal.WithLabelValues("otp", "throttled").Inc()
+		log.Printf("api: OTP mail refused by the install-wide mail budget (request_id=%s)", requestIDFromContext(ctx))
+		return errMailRateLimited(wait)
+	}
 	if err := a.Mailer.SendOTP(ctx, email, code); err != nil {
+		metrics.MailTotal.WithLabelValues("otp", "failed").Inc()
 		// Mapped here rather than at each of the four call sites, so every door that
 		// mails a code answers the same way. A relay refusal is neither the caller's
 		// fault nor a bug in Felis, and a bare 500 says neither — it reads as "the
@@ -269,6 +280,7 @@ func (a *API) deliverOTP(ctx context.Context, email, code string) error {
 		return newError(http.StatusBadGateway, "mail_undeliverable",
 			"the mail relay refused this message; ask the server operator to check the SMTP settings")
 	}
+	metrics.MailTotal.WithLabelValues("otp", "sent").Inc()
 	return nil
 }
 
diff --git a/internal/api/handlers_op_login.go b/internal/api/handlers_op_login.go
index 2e840ba..35c5110 100644
--- a/internal/api/handlers_op_login.go
+++ b/internal/api/handlers_op_login.go
@@ -86,6 +86,13 @@ func (a *API) handleOpLoginStart(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// The install-wide mail budget is checked before the address is resolved,
+	// so while it is spent every address gets the same 429.
+	if err := a.checkMailBudget(); err != nil {
+		writeError(w, r, err)
+		return
+	}
+
 	// Per-recipient cooldown reserved BEFORE any work, identical to the console email
 	// door: one winner per window, and the neutral (non-staff) branch keeps the
 	// reservation too so probing an address is throttled exactly like a real send. The
diff --git a/internal/api/handlers_passkey_discoverable.go b/internal/api/handlers_passkey_discoverable.go
index 564811e..db0c178 100644
--- a/internal/api/handlers_passkey_discoverable.go
+++ b/internal/api/handlers_passkey_discoverable.go
@@ -21,10 +21,10 @@ import (
 //
 // Anti-abuse divergence from the email-first door: that door reserves a per-recipient cooldown
 // (a.otpLimiter) keyed on the typed email. A usernameless begin has no recipient OR principal to
-// key a fair per-caller limit on, so — matching the stance in handlers_auth_email.go (behind
-// Cloudflare RemoteAddr is the proxy; CGNAT false-positives) — volumetric per-source limiting is
-// left to the edge, and the server-side bound is a hard global cap on live challenges enforced
-// atomically in CreateDiscoverableChallenge (ErrTooManyDiscoverableChallenges → 429).
+// key a fair per-caller limit on, so one client is bounded by the per-address token bucket every
+// public auth door sits behind (throttleAuthDoor), and the table by a hard global cap on live
+// challenges enforced atomically in CreateDiscoverableChallenge (ErrTooManyDiscoverableChallenges
+// → 429).
 
 // handlePasskeyLoginDiscoverableBegin starts a usernameless assertion ceremony (Public,
 // pre-session). It has no request body — the whole point is that the caller supplies no
diff --git a/internal/api/otp_lock.go b/internal/api/otp_lock.go
index d09e8e0..7189955 100644
--- a/internal/api/otp_lock.go
+++ b/internal/api/otp_lock.go
@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"log"
 	"net/http"
-	"strconv"
 	"time"
 
 	"felis.lolicon.best/internal/metrics"
@@ -35,14 +34,9 @@ var otpDoorName = map[string][2]string{
 
 // writeOTPAccountLocked answers a signed-in door whose budget is spent.
 func writeOTPAccountLocked(w http.ResponseWriter, r *http.Request, until, now time.Time) {
-	secs := int64(until.Sub(now).Round(time.Second) / time.Second)
-	if secs < 1 {
-		secs = 1
-	}
-	w.Header().Set("Retry-After", strconv.FormatInt(secs, 10))
 	writeError(w, r, newError(http.StatusTooManyRequests, "otp_account_locked",
 		"too many wrong codes on this account; email codes work again after %s",
-		until.UTC().Format(time.RFC3339)))
+		until.UTC().Format(time.RFC3339)).retryAfter(max(until.Sub(now), time.Second)))
 }
 
 // noteOTPLock handles a redeem that met the account lock. Only the guess that
@@ -83,10 +77,18 @@ func (a *API) noteOTPLock(r *http.Request, err error, userID, purpose string) {
 		log.Printf("auth: no notice mailer; user %s was not told their %s is locked", userID, purpose)
 		return
 	}
+	if ok, _ := a.mailGate().take(mailGateKey); !ok {
+		metrics.MailTotal.WithLabelValues("notice", "throttled").Inc()
+		log.Printf("auth: mail budget spent; user %s was not told their %s is locked", userID, purpose)
+		return
+	}
 	subject, body := otpLockNotice(door, lock.Until)
 	if err := sender.SendNotice(ctx, u.Email, subject, body); err != nil {
+		metrics.MailTotal.WithLabelValues("notice", "failed").Inc()
 		log.Printf("auth: otp lock notice to user %s failed: %v", userID, err)
+		return
 	}
+	metrics.MailTotal.WithLabelValues("notice", "sent").Inc()
 }
 
 // otpLockNotice renders the bilingual lock notice.
diff --git a/internal/api/otp_lock_test.go b/internal/api/otp_lock_test.go
index f01944d..606022d 100644
--- a/internal/api/otp_lock_test.go
+++ b/internal/api/otp_lock_test.go
@@ -3,9 +3,12 @@ package api
 import (
 	"context"
 	"net/http"
+	"slices"
 	"strings"
 	"testing"
 	"time"
+
+	"felis.lolicon.best/internal/metrics"
 )
 
 // The per-code attempt cap resets on every resend; these pin the account-level
@@ -181,3 +184,14 @@ func TestOTPLockNoticeNamesDoorAndTime(t *testing.T) {
 		}
 	}
 }
+
+// The lockout alert sees a purpose only if metrics pre-creates its series.
+func TestOTPPurposesMatchMetricLabels(t *testing.T) {
+	got := []string{otpPurposeOnboard, otpPurposeLogin, otpPurposeOpLogin, otpPurposeMigrate}
+	want := slices.Clone(metrics.OTPPurposes)
+	slices.Sort(got)
+	slices.Sort(want)
+	if !slices.Equal(got, want) {
+		t.Fatalf("otp purposes %v, metrics.OTPPurposes %v", got, want)
+	}
+}
diff --git a/internal/api/pgrepo.go b/internal/api/pgrepo.go
index 77e0467..6cd3a58 100644
--- a/internal/api/pgrepo.go
+++ b/internal/api/pgrepo.go
@@ -1218,9 +1218,8 @@ func (p *PGRepo) ConsumePasskeyChallengeByUser(ctx context.Context, userID, purp
 // exist, a new begin is refused (ErrTooManyDiscoverableChallenges → 429). The cap is generous —
 // a login challenge lives only passkeyChallengeTTL (5 min) and each row is ~1 KB — so real
 // concurrency never approaches it, while an abusive begin-flood is bounded to a few MB instead
-// of growing without limit. Volumetric per-IP limiting is the edge's job (handlers_auth_email.go):
-// behind Cloudflare RemoteAddr is the proxy, and a usernameless door has no recipient to key a
-// fair per-caller limit on.
+// of growing without limit. One client's volume is bounded before it gets here, by the
+// per-address token bucket in front of every public auth door (ratelimit.go).
 const maxLiveDiscoverableChallenges = 4096
 
 // CreateDiscoverableChallenge stashes a discoverable-login ceremony under an opaque handle,
diff --git a/internal/api/ratelimit.go b/internal/api/ratelimit.go
new file mode 100644
index 0000000..d5a50c7
--- /dev/null
+++ b/internal/api/ratelimit.go
@@ -0,0 +1,247 @@
+package api
+
+import (
+	"math"
+	"net"
+	"net/http"
+	"net/netip"
+	"strings"
+	"sync"
+	"time"
+
+	"felis.lolicon.best/internal/metrics"
+)
+
+// Volumetric limits for the public auth doors and for outbound mail.
+//
+// The per-recipient OTP cooldown (otpLimiter) stops one mailbox being bombed,
+// and the per-account wrong-code budget stops one account being guessed. Neither
+// bounds a caller who sprays many addresses or many accounts, so two more limits
+// sit in front of them:
+//
+//   - authDoorGate: a token bucket per client address over every pre-session
+//     auth door (options, login, op-login, bind, setup redeem). The client
+//     address comes from the edge's header only when the install says which
+//     header its edge writes (ClientIPHeader); with Cloudflare that header is
+//     CF-Connecting-IP, which is trustworthy because the edge setup fences the
+//     panel NodePort to loopback, so every request reaching the origin came
+//     through cloudflared. IPv6 clients are bucketed per /64, the unit one
+//     subscriber is handed.
+//   - mailGate: one install-wide bucket over every mail the API sends (codes
+//     and lock notices), so no flood can burn the SMTP relay's quota and get
+//     the sending account suspended. The public doors check it before they
+//     resolve the address, so a spent budget answers every address alike.
+
+// RateLimit is a token bucket: Burst calls at once, refilled at PerMinute. The
+// zero value disables the limit.
+type RateLimit struct {
+	Burst     int
+	PerMinute float64
+}
+
+func (l RateLimit) enabled() bool { return l.Burst > 0 && l.PerMinute > 0 }
+
+// bucketSweepEvery is how often idle buckets are dropped; bucketMaxKeys bounds
+// the map between sweeps. Past the bound, new keys share one overflow bucket,
+// so a spray of fresh source addresses throttles itself instead of growing the
+// map.
+const (
+	bucketSweepEvery = time.Minute
+	bucketMaxKeys    = 50_000
+	bucketOverflow   = "\x00overflow"
+)
+
+type tokenBucket struct {
+	tokens float64
+	at     time.Time
+}
+
+// bucketSet is a set of token buckets keyed by caller. A missing key is a full
+// bucket, so a bucket that has refilled completely carries no information and
+// is dropped by the sweep; memory is bounded by the keys active in the last
+// refill period.
+type bucketSet struct {
+	mu      sync.Mutex
+	now     func() time.Time
+	limit   RateLimit
+	buckets map[string]*tokenBucket
+	swept   time.Time
+	maxKeys int
+}
+
+func newBucketSet(limit RateLimit, now func() time.Time) *bucketSet {
+	return &bucketSet{now: now, limit: limit, buckets: map[string]*tokenBucket{}, maxKeys: bucketMaxKeys}
+}
+
+// refill brings b up to now. The caller holds mu.
+func (s *bucketSet) refill(b *tokenBucket, now time.Time) {
+	if elapsed := now.Sub(b.at); elapsed > 0 {
+		b.tokens = math.Min(float64(s.limit.Burst), b.tokens+elapsed.Minutes()*s.limit.PerMinute)
+	}
+	b.at = now
+}
+
+// sweep drops full buckets at most once per bucketSweepEvery, or at once when
+// force is set. The caller holds mu.
+func (s *bucketSet) sweep(now time.Time, force bool) {
+	if !force && now.Sub(s.swept) < bucketSweepEvery {
+		return
+	}
+	s.swept = now
+	for k, b := range s.buckets {
+		s.refill(b, now)
+		if b.tokens >= float64(s.limit.Burst) {
+			delete(s.buckets, k)
+		}
+	}
+}
+
+// bucket returns key's bucket, creating a full one. The caller holds mu.
+func (s *bucketSet) bucket(key string, now time.Time) *tokenBucket {
+	s.sweep(now, false)
+	if b, ok := s.buckets[key]; ok {
+		s.refill(b, now)
+		return b
+	}
+	if len(s.buckets) >= s.maxKeys {
+		s.sweep(now, true)
+		if len(s.buckets) >= s.maxKeys {
+			key = bucketOverflow
+			if b, ok := s.buckets[key]; ok {
+				s.refill(b, now)
+				return b
+			}
+		}
+	}
+	b := &tokenBucket{tokens: float64(s.limit.Burst), at: now}
+	s.buckets[key] = b
+	return b
+}
+
+// take spends one token from key's bucket. When none is left it reports how
+// long until one is. A disabled limit always admits.
+func (s *bucketSet) take(key string) (bool, time.Duration) {
+	if s == nil || !s.limit.enabled() {
+		return true, 0
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	now := s.now()
+	b := s.bucket(key, now)
+	if b.tokens >= 1 {
+		b.tokens--
+		return true, 0
+	}
+	return false, s.wait(b)
+}
+
+// peek reports whether key's bucket holds a token, without spending it.
+func (s *bucketSet) peek(key string) (bool, time.Duration) {
+	if s == nil || !s.limit.enabled() {
+		return true, 0
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	now := s.now()
+	b, ok := s.buckets[key]
+	if !ok {
+		return true, 0
+	}
+	s.refill(b, now)
+	if b.tokens >= 1 {
+		return true, 0
+	}
+	return false, s.wait(b)
+}
+
+// wait is how long until b holds one token. The caller holds mu.
+func (s *bucketSet) wait(b *tokenBucket) time.Duration {
+	missing := 1 - b.tokens
+	return time.Duration(math.Ceil(missing / s.limit.PerMinute * float64(time.Minute)))
+}
+
+func (a *API) authDoorGate() *bucketSet {
+	a.authDoorOnce.Do(func() { a.authDoorBuckets = newBucketSet(a.AuthDoorLimit, a.now) })
+	return a.authDoorBuckets
+}
+
+func (a *API) mailGate() *bucketSet {
+	a.mailOnce.Do(func() { a.mailBuckets = newBucketSet(a.MailLimit, a.now) })
+	return a.mailBuckets
+}
+
+// mailGateKey is the single install-wide mail bucket.
+const mailGateKey = "mail"
+
+// throttleAuthDoor applies the per-source bucket to one public auth door.
+func (a *API) throttleAuthDoor(h http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		ok, wait := a.authDoorGate().take(sourceKey(a.clientIP(r)))
+		if !ok {
+			metrics.RateLimitedTotal.WithLabelValues("auth_door").Inc()
+			writeError(w, r, newError(http.StatusTooManyRequests, "rate_limited",
+				"too many sign-in requests from this network; try again shortly").retryAfter(wait))
+			return
+		}
+		h(w, r)
+	}
+}
+
+// errMailRateLimited answers a door whose mail the install-wide budget refuses.
+func errMailRateLimited(wait time.Duration) *apiError {
+	return newError(http.StatusTooManyRequests, "mail_rate_limited",
+		"this server is sending too much mail right now; try again shortly").retryAfter(wait)
+}
+
+// checkMailBudget is the public doors' pre-resolution check: it refuses every
+// address alike while the budget is spent, so the refusal says nothing about
+// whether the address has an account.
+func (a *API) checkMailBudget() error {
+	if a.Mailer == nil {
+		return nil
+	}
+	if ok, wait := a.mailGate().peek(mailGateKey); !ok {
+		metrics.MailTotal.WithLabelValues("otp", "throttled").Inc()
+		return errMailRateLimited(wait)
+	}
+	return nil
+}
+
+// clientIP is the caller's address: the edge's header when the install names
+// one and the request carries a parseable value, else the TCP peer. For
+// X-Forwarded-For the rightmost entry is used, the one the trusted proxy
+// appended itself.
+func (a *API) clientIP(r *http.Request) netip.Addr {
+	if name := a.ClientIPHeader; name != "" {
+		if v := r.Header.Get(name); v != "" {
+			if strings.EqualFold(name, "X-Forwarded-For") {
+				if i := strings.LastIndexByte(v, ','); i >= 0 {
+					v = v[i+1:]
+				}
+			}
+			if ip, err := netip.ParseAddr(strings.TrimSpace(v)); err == nil {
+				return ip.Unmap()
+			}
+		}
+	}
+	host, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		host = r.RemoteAddr
+	}
+	ip, _ := netip.ParseAddr(host)
+	return ip.Unmap()
+}
+
+// sourceKey buckets an address: IPv4 per host, IPv6 per /64. An unparseable
+// address shares one key.
+func sourceKey(ip netip.Addr) string {
+	switch {
+	case !ip.IsValid():
+		return "unknown"
+	case ip.Is4():
+		return ip.String()
+	default:
+		p, _ := ip.Prefix(64)
+		return p.String()
+	}
+}
diff --git a/internal/api/ratelimit_test.go b/internal/api/ratelimit_test.go
new file mode 100644
index 0000000..82d4b67
--- /dev/null
+++ b/internal/api/ratelimit_test.go
@@ -0,0 +1,269 @@
+package api
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"strings"
+	"testing"
+	"time"
+)
+
+// These pin the two volumetric limits in front of the public sign-in doors:
+// the per-client-address bucket (keyed on the edge's visitor header only when
+// the install names it) and the install-wide mail budget, which must refuse
+// every address alike so it never becomes an existence oracle. They also pin
+// that neither the buckets nor the OTP cooldown map grow without bound.
+
+func TestBucketSetBurstRefillAndWait(t *testing.T) {
+	clock := time.Unix(1_700_000_000, 0)
+	s := newBucketSet(RateLimit{Burst: 3, PerMinute: 6}, func() time.Time { return clock })
+	for i := 0; i < 3; i++ {
+		if ok, _ := s.take("a"); !ok {
+			t.Fatalf("take %d refused inside the burst", i+1)
+		}
+	}
+	ok, wait := s.take("a")
+	if ok || wait != 10*time.Second {
+		t.Fatalf("4th take = %v wait %v, want refused with 10s wait (6/min)", ok, wait)
+	}
+	if ok, _ := s.take("b"); !ok {
+		t.Fatal("another key shares a's bucket")
+	}
+	if ok, _ := s.peek("a"); ok {
+		t.Fatal("peek admitted an empty bucket")
+	}
+	clock = clock.Add(10 * time.Second)
+	if ok, _ := s.peek("a"); !ok {
+		t.Fatal("peek refused after one token refilled")
+	}
+	if ok, _ := s.take("a"); !ok {
+		t.Fatal("take refused after one token refilled (peek must not spend it)")
+	}
+	if ok, _ := s.take("a"); ok {
+		t.Fatal("a second token appeared from nowhere")
+	}
+}
+
+func TestBucketSetDropsIdleKeysAndCapsTheMap(t *testing.T) {
+	clock := time.Unix(1_700_000_000, 0)
+	s := newBucketSet(RateLimit{Burst: 2, PerMinute: 60}, func() time.Time { return clock })
+	s.maxKeys = 100
+	for i := 0; i < 100; i++ {
+		s.take(fmt.Sprintf("10.0.0.%d", i))
+	}
+	// At the cap with every bucket still draining: fresh keys share the
+	// overflow bucket instead of growing the map.
+	s.take("fresh-1")
+	s.take("fresh-2")
+	if ok, _ := s.take("fresh-3"); ok {
+		t.Fatal("overflow bucket admitted past its burst")
+	}
+	if n := len(s.buckets); n != 101 {
+		t.Fatalf("map holds %d buckets, want 100 + overflow", n)
+	}
+	// Once they refill, the sweep drops them all.
+	clock = clock.Add(2 * bucketSweepEvery)
+	s.take("later")
+	if n := len(s.buckets); n != 1 {
+		t.Fatalf("after refill the map holds %d buckets, want only the new one", n)
+	}
+}
+
+func TestDisabledLimitAdmitsEverything(t *testing.T) {
+	var nilSet *bucketSet
+	if ok, _ := nilSet.take("x"); !ok {
+		t.Fatal("nil set refused")
+	}
+	s := newBucketSet(RateLimit{}, time.Now)
+	for i := 0; i < 1000; i++ {
+		if ok, _ := s.take("x"); !ok {
+			t.Fatal("zero limit refused")
+		}
+	}
+}
+
+func TestClientIPTrustsOnlyTheNamedHeader(t *testing.T) {
+	req := func(remote string, h map[string]string) *http.Request {
+		r := httptest.NewRequest("POST", "/", nil)
+		r.RemoteAddr = remote
+		for k, v := range h {
+			r.Header.Set(k, v)
+		}
+		return r
+	}
+	for _, tc := range []struct {
+		name   string
+		header string
+		r      *http.Request
+		want   string
+	}{
+		{"no header configured ignores CF-Connecting-IP", "", req("10.42.0.1:5000", map[string]string{"CF-Connecting-IP": "203.0.113.9"}), "10.42.0.1"},
+		{"cloudflare header", "CF-Connecting-IP", req("10.42.0.1:5000", map[string]string{"CF-Connecting-IP": "203.0.113.9"}), "203.0.113.9"},
+		{"cloudflare header absent falls back to peer", "CF-Connecting-IP", req("10.42.0.1:5000", nil), "10.42.0.1"},
+		{"garbage header falls back to peer", "CF-Connecting-IP", req("10.42.0.1:5000", map[string]string{"CF-Connecting-IP": "not-an-ip"}), "10.42.0.1"},
+		{"X-Forwarded-For takes the proxy-appended rightmost hop", "X-Forwarded-For", req("10.42.0.1:5000", map[string]string{"X-Forwarded-For": "1.1.1.1, 198.51.100.7"}), "198.51.100.7"},
+		{"v4-mapped v6 is unmapped", "CF-Connecting-IP", req("10.42.0.1:5000", map[string]string{"CF-Connecting-IP": "::ffff:203.0.113.9"}), "203.0.113.9"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			a := &API{ClientIPHeader: tc.header}
+			if got := a.clientIP(tc.r).String(); got != tc.want {
+				t.Fatalf("clientIP = %s, want %s", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSourceKeyGroupsIPv6By64(t *testing.T) {
+	a := sourceKey(netip.MustParseAddr("2001:db8:1:2::1"))
+	b := sourceKey(netip.MustParseAddr("2001:db8:1:2:ffff::9"))
+	c := sourceKey(netip.MustParseAddr("2001:db8:1:3::1"))
+	if a != b || a == c {
+		t.Fatalf("keys %q %q %q: want one per /64", a, b, c)
+	}
+	if k := sourceKey(netip.MustParseAddr("203.0.113.9")); k != "203.0.113.9" {
+		t.Fatalf("v4 key = %q", k)
+	}
+}
+
+func TestAuthDoorsThrottlePerClientAddress(t *testing.T) {
+	api, _, _ := seedLoginEmailAPI(t)
+	api.AuthDoorLimit = RateLimit{Burst: 3, PerMinute: 3}
+	api.ClientIPHeader = "CF-Connecting-IP"
+	eh := api.ExternalHandler()
+	from := func(ip string) map[string]string {
+		return map[string]string{"Content-Type": "application/json", "CF-Connecting-IP": ip}
+	}
+	for i := 0; i < 3; i++ {
+		if w := do(eh, "POST", "/api/v1/auth/options", `{"email":"x@example.net"}`, from("203.0.113.9")); w.Code != http.StatusOK {
+			t.Fatalf("call %d = %d (%s)", i+1, w.Code, w.Body.String())
+		}
+	}
+	// The bucket spans every door: a different door from the same address is
+	// refused too.
+	w := do(eh, "POST", "/api/v1/auth/email/verify", `{"email":"x@example.net","code":"000000"}`, from("203.0.113.9"))
+	if c, _ := errEnvelope(t, w); w.Code != http.StatusTooManyRequests || c != "rate_limited" {
+		t.Fatalf("4th call = %d %s, want 429 rate_limited", w.Code, c)
+	}
+	if ra := w.Header().Get("Retry-After"); ra != "20" {
+		t.Fatalf("Retry-After = %q, want 20 (3/min)", ra)
+	}
+	if w := do(eh, "POST", "/api/v1/auth/options", `{"email":"x@example.net"}`, from("198.51.100.7")); w.Code != http.StatusOK {
+		t.Fatalf("another address was throttled: %d", w.Code)
+	}
+	// The op-login poll and logout are not doors: a waiting browser polls.
+	for i := 0; i < 5; i++ {
+		if w := do(eh, "GET", "/api/v1/auth/op-login/status/abc", "", from("203.0.113.9")); w.Code == http.StatusTooManyRequests {
+			t.Fatal("op-login status poll was throttled")
+		}
+	}
+}
+
+func TestAuthDoorsIgnoreVisitorHeaderUnlessConfigured(t *testing.T) {
+	api, _, _ := seedLoginEmailAPI(t)
+	api.AuthDoorLimit = RateLimit{Burst: 2, PerMinute: 2}
+	eh := api.ExternalHandler()
+	// Without client_ip_header a forged CF-Connecting-IP must not mint fresh
+	// buckets: every call below comes from httptest's one peer address.
+	for i := 0; i < 2; i++ {
+		do(eh, "POST", "/api/v1/auth/options", `{"email":"x@example.net"}`,
+			map[string]string{"Content-Type": "application/json", "CF-Connecting-IP": fmt.Sprintf("203.0.113.%d", i)})
+	}
+	w := do(eh, "POST", "/api/v1/auth/options", `{"email":"x@example.net"}`,
+		map[string]string{"Content-Type": "application/json", "CF-Connecting-IP": "203.0.113.200"})
+	if w.Code != http.StatusTooManyRequests {
+		t.Fatalf("forged visitor header escaped the limit: %d", w.Code)
+	}
+}
+
+func TestMailBudgetRefusesEveryAddressAlike(t *testing.T) {
+	api, repo, mailer := seedLoginEmailAPI(t)
+	repo.staff["second"] = &StaffUser{ID: "u2", Username: "second", Email: "second@example.net", Role: "user", EmailVerified: true}
+	api.MailLimit = RateLimit{Burst: 1, PerMinute: 1}
+	clock := time.Unix(1_700_000_000, 0)
+	api.Now = func() time.Time { return clock }
+	eh := api.ExternalHandler()
+	start := func(email string) *httptest.ResponseRecorder {
+		return do(eh, "POST", "/api/v1/auth/email/start", `{"email":"`+email+`"}`, jsonHeader)
+	}
+
+	if w := start("player@example.net"); w.Code != http.StatusAccepted || mailer.calls != 1 {
+		t.Fatalf("first send = %d, %d mails", w.Code, mailer.calls)
+	}
+	// Budget spent: a real account and an unknown address get the same 429,
+	// and nothing reaches the relay.
+	for _, email := range []string{"second@example.net", "nobody@example.net"} {
+		w := start(email)
+		if c, _ := errEnvelope(t, w); w.Code != http.StatusTooManyRequests || c != "mail_rate_limited" || w.Header().Get("Retry-After") == "" {
+			t.Fatalf("%s while the budget is spent = %d %s (Retry-After %q)", email, w.Code, c, w.Header().Get("Retry-After"))
+		}
+	}
+	if mailer.calls != 1 {
+		t.Fatalf("a spent budget still mailed: %d sends", mailer.calls)
+	}
+	// The refused starts did not burn their recipients' cooldowns.
+	clock = clock.Add(time.Minute)
+	if w := start("second@example.net"); w.Code != http.StatusAccepted || mailer.calls != 2 {
+		t.Fatalf("after refill = %d, %d mails", w.Code, mailer.calls)
+	}
+}
+
+func TestSignedInDoorMailBudget(t *testing.T) {
+	repo := newFakeRepo()
+	repo.staff["player"] = &StaffUser{ID: "u1", Username: "player"}
+	api := newTestAPI(repo, newFakeCluster())
+	api.External = staticExternal{p: &Principal{UserID: "u1", Email: "u1@example.net", Role: "user"}}
+	mailer := &captureMailer{}
+	api.Mailer = mailer
+	// One mail per two minutes, so stepping past the per-principal resend
+	// cooldown leaves the mail budget still empty.
+	api.MailLimit = RateLimit{Burst: 1, PerMinute: 0.5}
+	clock := time.Unix(1_700_000_000, 0)
+	api.Now = func() time.Time { return clock }
+	eh := api.ExternalHandler()
+	if w := do(eh, "POST", "/api/v1/account/email/start", `{"email":"a@example.net"}`, nil); w.Code != http.StatusAccepted {
+		t.Fatalf("first = %d (%s)", w.Code, w.Body.String())
+	}
+	clock = clock.Add(otpResendCooldown + time.Second)
+	w := do(eh, "POST", "/api/v1/account/email/start", `{"email":"b@example.net"}`, nil)
+	if c, _ := errEnvelope(t, w); w.Code != http.StatusTooManyRequests || c != "mail_rate_limited" {
+		t.Fatalf("second = %d %s, want 429 mail_rate_limited", w.Code, c)
+	}
+	if mailer.calls != 1 {
+		t.Fatalf("mails = %d", mailer.calls)
+	}
+}
+
+func TestCooldownLimiterForgetsExpiredKeys(t *testing.T) {
+	clock := time.Unix(1_700_000_000, 0)
+	c := &cooldownLimiter{now: func() time.Time { return clock }, last: map[string]time.Time{}}
+	for i := 0; i < 500; i++ {
+		if _, ok := c.reserve(fmt.Sprintf("login:email:user%d@example.net", i), time.Minute); !ok {
+			t.Fatalf("reserve %d refused", i)
+		}
+	}
+	if _, ok := c.reserve("login:email:user7@example.net", time.Minute); ok {
+		t.Fatal("a live reservation was forgotten")
+	}
+	clock = clock.Add(time.Minute + bucketSweepEvery)
+	c.reserve("login:email:fresh@example.net", time.Minute)
+	if n := len(c.last); n != 1 {
+		t.Fatalf("after every window ended the map holds %d keys, want 1", n)
+	}
+}
+
+func TestMailLimitCountsNotices(t *testing.T) {
+	// The lock notice spends the same budget as codes; with none left it is
+	// skipped rather than sent.
+	api, _, _ := seedLoginEmailAPI(t)
+	mailer := &noticeMailer{}
+	api.Mailer = mailer
+	api.MailLimit = RateLimit{Burst: 1, PerMinute: 1}
+	api.mailGate().take(mailGateKey)
+	r := httptest.NewRequest("POST", "/", strings.NewReader(""))
+	api.noteOTPLock(r, &OTPAccountLockedError{Until: api.now().Add(time.Hour), JustLocked: true}, "u1", otpPurposeLogin)
+	if len(mailer.notices) != 0 {
+		t.Fatalf("notice sent past a spent budget: %q", mailer.notices)
+	}
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index c6fae61..b0f86fd 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -71,8 +71,17 @@ type SMTPConfig struct {
 	// Username is the AUTH identity; empty means the relay needs no AUTH.
 	Username    string `toml:"username"`
 	PasswordRef string `toml:"password_ref"`
+	// MaxPerHour caps the mail the API sends install-wide (codes and notices),
+	// so a flood cannot spend the relay's quota and get the account suspended.
+	// 0 means DefaultMailPerHour. Size it to the relay's own limit.
+	MaxPerHour int `toml:"max_per_hour"`
 }
 
+// DefaultMailPerHour is the install-wide mail cap when smtp.max_per_hour is
+// unset: far above a community's normal sign-in mail, far below the daily
+// quota of common relays.
+const DefaultMailPerHour = 120
+
 // ServerConfig is the [server] table.
 type ServerConfig struct {
 	Listen     string `toml:"listen"`
@@ -106,6 +115,24 @@ type AuthConfig struct {
 	AdminHostname string `toml:"admin_hostname"`
 	PanelHostname string `toml:"panel_hostname"`
 	AccessJWTAud  string `toml:"access_jwt_aud"`
+	// ClientIPHeader names the header the edge writes the visitor's address
+	// into: CF-Connecting-IP behind the Cloudflare tunnel (the edge setup
+	// writes it), X-Forwarded-For behind an operator's reverse proxy. The API
+	// keys its per-client sign-in rate limit on it. Empty means the TCP peer,
+	// except that an install with an Access audience (set only by the
+	// Cloudflare edge setup) implies CF-Connecting-IP.
+	ClientIPHeader string `toml:"client_ip_header"`
+}
+
+// EffectiveClientIPHeader resolves ClientIPHeader with its Cloudflare default.
+func (a AuthConfig) EffectiveClientIPHeader() string {
+	if a.ClientIPHeader != "" {
+		return a.ClientIPHeader
+	}
+	if a.AccessJWTAud != "" {
+		return "CF-Connecting-IP"
+	}
+	return ""
 }
 
 // K8sConfig is the [k8s] table.
@@ -368,6 +395,12 @@ func (c *Config) Validate() error {
 			return fmt.Errorf("config: [smtp] port %d must be 1-65535 (587 STARTTLS, 465 implicit TLS)", c.SMTP.Port)
 		}
 	}
+	if c.SMTP.MaxPerHour < 0 {
+		return fmt.Errorf("config: [smtp] max_per_hour %d must be positive (0 means the default %d)", c.SMTP.MaxPerHour, DefaultMailPerHour)
+	}
+	if h := c.Auth.ClientIPHeader; strings.ContainsAny(h, " :\t\r\n") {
+		return fmt.Errorf("config: [auth] client_ip_header %q must be a bare header name such as CF-Connecting-IP or X-Forwarded-For", h)
+	}
 	return c.validateAuthSources()
 }
 
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index d6d69a4..cf42df1 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -574,3 +574,38 @@ host = "smtp.example.net"
 		t.Fatal("expected error when [smtp] host is set without a from address")
 	}
 }
+
+// TestClientIPHeaderAndMailCap pins the two knobs behind the sign-in rate
+// limits: the visitor-address header (explicit, or implied by an Access
+// audience that only the Cloudflare edge setup writes) and the mail cap.
+func TestClientIPHeaderAndMailCap(t *testing.T) {
+	base := `
+[server]
+root_domain = "mc.example.net"
+[database]
+url = "postgres://felis@db/felis"
+`
+	for _, tc := range []struct {
+		name, auth, want string
+	}{
+		{"unset", "", ""},
+		{"cloudflare audience implies CF-Connecting-IP", "access_jwt_aud = \"aud123\"\n", "CF-Connecting-IP"},
+		{"explicit wins", "access_jwt_aud = \"aud123\"\nclient_ip_header = \"X-Forwarded-For\"\n", "X-Forwarded-For"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg, err := config.Load(writeTOML(t, base+"[auth]\n"+tc.auth))
+			if err != nil {
+				t.Fatalf("Load: %v", err)
+			}
+			if got := cfg.Auth.EffectiveClientIPHeader(); got != tc.want {
+				t.Fatalf("EffectiveClientIPHeader = %q, want %q", got, tc.want)
+			}
+		})
+	}
+	if _, err := config.Load(writeTOML(t, base+"[auth]\nclient_ip_header = \"CF-Connecting-IP: 1.2.3.4\"\n")); err == nil {
+		t.Fatal("a header line with a value was accepted as a header name")
+	}
+	if _, err := config.Load(writeTOML(t, base+"[smtp]\nhost = \"smtp.example.net\"\nfrom = \"f@example.net\"\nmax_per_hour = -1\n")); err == nil {
+		t.Fatal("negative max_per_hour accepted")
+	}
+}
diff --git a/internal/metrics/metrics.go b/internal/metrics/metrics.go
index 0baf7fc..5a4d5b7 100644
--- a/internal/metrics/metrics.go
+++ b/internal/metrics/metrics.go
@@ -64,8 +64,44 @@ var (
 		Name:      "auth_otp_lockouts_total",
 		Help:      "Email-code doors locked after too many wrong codes, by purpose.",
 	}, []string{"purpose"})
+
+	// MailTotal counts mail the API tried to send, by kind (otp, notice) and
+	// result: sent, failed (the relay refused it) or throttled (the
+	// install-wide mail budget refused it before it reached the relay).
+	MailTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: namespace,
+		Name:      "mail_total",
+		Help:      "Mail the API tried to send, by kind and result (sent, failed, throttled).",
+	}, []string{"kind", "result"})
+
+	// RateLimitedTotal counts requests refused by a volumetric limit, by scope
+	// (auth_door: one client address calling the public sign-in doors too fast).
+	RateLimitedTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: namespace,
+		Name:      "rate_limited_total",
+		Help:      "Requests refused by a volumetric rate limit, by scope.",
+	}, []string{"scope"})
 )
 
+// OTPPurposes are the email-code doors OTPLockoutsTotal is labelled by.
+var OTPPurposes = []string{"onboard_email", "login_email", "op_login", "migrate_confirm"}
+
+// The sign-in alerts watch these counters with increase(). A labelled child
+// that does not exist yet has no sample before its first event, so increase()
+// would miss exactly the first lockout or throttle; every child the alerts use
+// is created at zero up front.
+func init() {
+	for _, kind := range []string{"otp", "notice"} {
+		for _, result := range []string{"sent", "failed", "throttled"} {
+			MailTotal.WithLabelValues(kind, result)
+		}
+	}
+	RateLimitedTotal.WithLabelValues("auth_door")
+	for _, p := range OTPPurposes {
+		OTPLockoutsTotal.WithLabelValues(p)
+	}
+}
+
 // SyncServerGauge republishes felis_servers_total from a full snapshot of the
 // fleet's per-server states. states holds one entry per MinecraftServer the
 // operator knows about (its desiredState).
@@ -97,6 +133,8 @@ func Collectors() []prometheus.Collector {
 		ImageBuildFailuresTotal,
 		ReaperWorldsDeletedTotal,
 		OTPLockoutsTotal,
+		MailTotal,
+		RateLimitedTotal,
 	}
 }
 
diff --git a/internal/metrics/metrics_test.go b/internal/metrics/metrics_test.go
index 42bbade..9a96cbc 100644
--- a/internal/metrics/metrics_test.go
+++ b/internal/metrics/metrics_test.go
@@ -116,3 +116,26 @@ func TestCountersRecordExpectedValues(t *testing.T) {
 		t.Errorf("start_duration_seconds collected %d metrics, want 1 histogram", n)
 	}
 }
+
+// The sign-in alerts use increase(), which needs a zero sample before the first
+// event; every child they watch must be exposed before anything is counted.
+func TestSignInSeriesStartAtZero(t *testing.T) {
+	want := map[string]int{
+		"felis_mail_total":              6,
+		"felis_rate_limited_total":      1,
+		"felis_auth_otp_lockouts_total": len(OTPPurposes),
+	}
+	for _, c := range []prometheus.Collector{MailTotal, RateLimitedTotal, OTPLockoutsTotal} {
+		reg := prometheus.NewRegistry()
+		reg.MustRegister(c)
+		mfs, err := reg.Gather()
+		if err != nil {
+			t.Fatalf("Gather: %v", err)
+		}
+		for _, mf := range mfs {
+			if n := len(mf.GetMetric()); n < want[mf.GetName()] {
+				t.Errorf("%s exposes %d children, want at least %d", mf.GetName(), n, want[mf.GetName()])
+			}
+		}
+	}
+}
diff --git a/panel/src/i18n/resources/en-US/errors.json b/panel/src/i18n/resources/en-US/errors.json
index 8656c93..c96b614 100644
--- a/panel/src/i18n/resources/en-US/errors.json
+++ b/panel/src/i18n/resources/en-US/errors.json
@@ -23,6 +23,8 @@
   "otp_resend_cooldown": "Verification code requested too frequently, please try again later.",
   "otp_locked": "Too many incorrect attempts, please request a new verification code.",
   "otp_account_locked": "Too many wrong codes in 24 hours — email codes for this account are paused. Try again later or use a passkey.",
+  "rate_limited": "Too many sign-in attempts from your network — wait a minute and try again.",
+  "mail_rate_limited": "This server is sending too much mail right now, so codes are paused. Try again shortly or use a passkey.",
   "passkey_challenge_invalid": "Passkey challenge is invalid or expired, please try again.",
   "invalid_attestation": "Could not verify this Passkey, please try again.",
   "passkey_already_bound": "This Passkey is already bound to another account.",
diff --git a/panel/src/i18n/resources/zh-CN/errors.json b/panel/src/i18n/resources/zh-CN/errors.json
index c1c8d47..725534f 100644
--- a/panel/src/i18n/resources/zh-CN/errors.json
+++ b/panel/src/i18n/resources/zh-CN/errors.json
@@ -23,6 +23,8 @@
   "otp_resend_cooldown": "验证码发送频繁，请稍后再试。",
   "otp_locked": "验证码错误次数过多，请重新获取验证码。",
   "otp_account_locked": "24 小时内验证码错误次数已达上限，此账号的邮箱验证码暂时停用，请稍后再试或改用 Passkey。",
+  "rate_limited": "登录请求过于频繁，请稍等一分钟再试。",
+  "mail_rate_limited": "服务器当前发信过多，验证码暂时发不出去，请稍后再试或改用 Passkey。",
   "passkey_challenge_invalid": "验证挑战无效或已过期，请重试。",
   "invalid_attestation": "无法验证此 Passkey，请重试。",
   "passkey_already_bound": "此 Passkey 已被其他账户绑定。",
diff --git a/panel/src/lib/api.ts b/panel/src/lib/api.ts
index b057924..b2fd60f 100644
--- a/panel/src/lib/api.ts
+++ b/panel/src/lib/api.ts
@@ -679,6 +679,12 @@ export function humanizeError(e: unknown): string {
       return t("otp_locked");
     case "otp_account_locked":
       return t("otp_account_locked");
+    // Volumetric limits on the sign-in doors (internal/api/ratelimit.go): one
+    // network calling too fast, or the install-wide mail budget spent.
+    case "rate_limited":
+      return t("rate_limited");
+    case "mail_rate_limited":
+      return t("mail_rate_limited");
     case "passkey_challenge_invalid":
       return t("passkey_challenge_invalid");
     case "invalid_attestation":