docs(mail): say plainly what a green SMTP self-test does not prove

// NOOP-only Ping green-lit a From on a domain the account could not send as —

// every OTP after it died at w.Close() with the wizard reporting success.

//

// Addressing the self-test to From cannot cause a false negative: this is an

// authenticated submission relay, whose job is to accept RCPT for any

// destination, so the recipient is never what a refusal is about — while the

// sender identity, which is, still gets checked. It also puts the proof

// somewhere the operator can go look at it.

// What it does NOT prove is that the relay will let this From reach anyone

// else. The self-test is addressed to From, which is a mailbox inside the

// relay account, and a relay that gates sender identity at end-of-DATA gates

// it on the way OUT: Fastmail answers 250 for [email protected] → the

// account's own mailbox and 551 5.7.1 "Not authorised to send from this

// header address" for that same From → any external recipient. Only the

// account's exact authorized identity passes the second one — another

// local-part on the same domain is refused too. So a green Ping means

// connect/TLS/AUTH/message-shape are good; the operator still has to have

// authorized From as a sending identity with their provider, and the first

// player OTP is what proves they did. Addressing the self-test elsewhere

// would not fix this — the only mailbox an operator can check is usually

// inside the account as well — so the honest move is to say so here rather

// than to buy false confidence with a bigger probe.

func (s *SMTP) Ping(ctx context.Context) error {

	c, err := s.connect(ctx)

	if err != nil {