Security audit 2026-10-17, finding S6 (low), stage 0 - proven from
outside: `ETRN <an inbound domain>` on port 25 answered "250 Queuing
started". smtpd_etrn_restrictions was empty (Postfix allows every client)
and fast_flush_domains = $relay_domains, so a stranger could flush the
deferred queue toward an inbound domain's upstream at will. Now
smtpd_etrn_restrictions=reject (etrn_cmd -> smtpd_check_etrn -> reject_code
554, Postfix 3.10.13 src/smtpd/smtpd.c) and smtpd_discard_ehlo_keywords=etrn.
e2e: testInboundRelay sends EHLO + ETRN for the inbound domain on port 25
and expects 554 and no ETRN in the EHLO reply (etrnPort25);
checkPostfixHardening reads both settings back. To be run on the server.
Docs: architecture.md § Postfix; CHANGELOG; plan.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
mixandClaude Opus 5.5
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