The problem does indeed come from Discourse.
By refining the log settings, I see that the posted message is correctly authenticated by the right address:
2026-02-25T12:54:24Z TRACE Data store
iteration operation (store.data-iterate) elapsed = 0ms
<3.14, remotePort = 47104, accountName =
“forum.cagut@domain.tld”, accountId = 31
But it becomes
2026-02-25T12:54:24Z [[35;1mTRACE [[37;1mRaw SMTP output sent
(smtp.raw-output) listenerId = “submissions”, localPort = 465, remoteIp
= 51.178.53.14, >
<53.14, remotePort = 47104, size = 73,
contents = “MAIL
FROM:<reply.cagut+verp-8bf40c695b1f612e555cf25f3c14c9b8@domain.tld>\\r\\n”
The mail server has no knowledge of this address format.