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/.env.example

@@ -11,6 +11,21 @@ SELFPOST_HOSTNAME=mail.example.com
 # primary 465/smtps listener.
 SUBMISSION_ENABLE=false
 
+# Optional inbound relay (backup-MX / forwarder). When true, Postfix accepts
+# mail on port 25 only for domains configured in the panel and forwards them
+# to the upstream you set. Off by default — the outbound path is unchanged.
+INBOUND_RELAY_ENABLE=false
+
+# Optional milter on the inbound listener only (not 465/587). Empty = off.
+# Example with deploy/antispam/docker-compose.antispam.yml:
+# INBOUND_ANTISPAM_MILTER=inet:antispam:11332
+# INBOUND_ANTISPAM_MILTER_ACTION=accept
+
+# Coarse per-client-IP limit and message size on inbound smtpd (port 25).
+# INBOUND_RATE_LIMIT_MESSAGES_PER_IP=20
+# INBOUND_MESSAGE_SIZE_LIMIT=26214400
+
+
 # Level-1 backstop rate limit (anvil) — see docs/guide.md "Rate limiting".
 RATE_LIMIT_MESSAGES_PER_IP=100
 RATE_LIMIT_WINDOW_SECONDS=3600