Add optional Phase O1 (targeted at the 2.x.x release line, outside the
v1.0 baseline) covering inbound relay as an opt-in/plugin: accept on :25
for explicit relay_domains and forward to an upstream backend, with
strict anti-open-relay/backscatter (relay_domains + relay_recipient_maps
+ reject_unauth_destination). Use cases: backup-MX and fronting a mail
server with no external IP.
Antispam is an important but optional capability: blind forwarding stays
valid. Since a blind relay hides the origin IP from the backend (breaking
downstream DNSBL/SPF), filtering must be attachable at the inbound hop —
provided as a milter hook to an external engine running in a separate
optional container, plus native Postfix DNSBL as a dependency-free
backstop. SelfPost neither bundles nor runs the engine, keeping the image
and the "one container, three processes" model intact.
Requires explicit sign-off (spec 12.6) as it extends beyond out-of-scope
section 3; plan-only, no implementation.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
mix
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