Add optional inbound relay (backup-MX) behind INBOUND_RELAY_ENABLE.

Port 25 accepts only configured domains and listed recipients, then forwards to an upstream; the outbound path is unchanged when the flag is off.

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/nginx/docker-compose.nginx.yml

@@ -23,6 +23,7 @@ services:
     ports: !override
       - "465:465"
       - "587:587"
+      - "25:25"
       # No host publish for 8080 here: nginx reaches it over the compose
       # network at selfpost:8080 instead (see nginx.conf.example).
     volumes: !override