The Ping comment claimed the self-test could not produce a false negative and
left the impression it therefore proved deliverability. It does not, and the
distinction is the whole trap: a relay that gates sender identity at
end-of-DATA gates it on the way OUT. Fastmail answers 250 for
[email protected] addressed to the account's own mailbox and 551 5.7.1 "Not
authorised to send from this header address" for that same From addressed to
anyone else — and 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 and message shape are good, and
nothing more; the operator still has to have authorized From as a sending
identity with their provider, and the first real OTP is what proves they did.
Addressing the self-test somewhere external would not fix it either — the only
mailbox an operator can reliably check is usually inside the same account — so
the honest move is to scope the claim rather than buy false confidence with a
bigger probe.
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