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

deploy/antispam/docker-compose.antispam.yml

@@ -0,0 +1,29 @@
+# Optional anti-spam sidecar for inbound relay (docs/plans/inbound-relay.md).
+#
+# SelfPost does not ship or start an anti-spam engine. This fragment brings up
+# rspamd next to SelfPost and points the inbound smtpd milter at it. Merge it
+# only when you want filtering on port 25:
+#
+#   docker compose -f docker-compose.yml -f antispam/docker-compose.antispam.yml up -d
+#
+# Requires INBOUND_RELAY_ENABLE=true in .env. The milter sees the real client
+# IP, HELO and PTR — unlike the upstream, which only sees SelfPost's address.
+#
+# Pin the rspamd tag the same way the SelfPost image is pinned; bump it
+# deliberately. Default milter action is fail-open (accept) so a down sidecar
+# does not block backup-MX. Set INBOUND_ANTISPAM_MILTER_ACTION=tempfail to
+# defer instead.
+
+services:
+  selfpost:
+    environment:
+      INBOUND_RELAY_ENABLE: "${INBOUND_RELAY_ENABLE:-true}"
+      INBOUND_ANTISPAM_MILTER: "${INBOUND_ANTISPAM_MILTER:-inet:antispam:11332}"
+      INBOUND_ANTISPAM_MILTER_ACTION: "${INBOUND_ANTISPAM_MILTER_ACTION:-accept}"
+    depends_on:
+      - antispam
+
+  antispam:
+    image: rspamd/rspamd:3.11
+    restart: unless-stopped
+    # No host ports: only SelfPost talks to the milter on the compose network.

deploy/caddy/docker-compose.caddy.yml

@@ -23,6 +23,7 @@ services:
     ports: !override
       - "465:465"
       - "587:587"
+      - "25:25"
     environment:
       # Caddy names certificate files after the hostname, not
       # fullchain.pem/privkey.pem like certbot — override the base file's

deploy/docker-compose.yml

@@ -33,6 +33,11 @@ services:
       # Set to true to also publish RFC 6409 submission (587/STARTTLS)
       # alongside the primary 465/smtps listener (spec 5).
       SUBMISSION_ENABLE: "${SUBMISSION_ENABLE:-false}"
+      INBOUND_RELAY_ENABLE: "${INBOUND_RELAY_ENABLE:-false}"
+      INBOUND_ANTISPAM_MILTER: "${INBOUND_ANTISPAM_MILTER:-}"
+      INBOUND_ANTISPAM_MILTER_ACTION: "${INBOUND_ANTISPAM_MILTER_ACTION:-accept}"
+      INBOUND_RATE_LIMIT_MESSAGES_PER_IP: "${INBOUND_RATE_LIMIT_MESSAGES_PER_IP:-20}"
+      INBOUND_MESSAGE_SIZE_LIMIT: "${INBOUND_MESSAGE_SIZE_LIMIT:-26214400}"
       # Level-1 backstop rate limit (anvil, spec 5.5, 7.4); per-domain/app
       # limits (level 2) are configured later from the panel itself.
       RATE_LIMIT_MESSAGES_PER_IP: "${RATE_LIMIT_MESSAGES_PER_IP:-100}"
@@ -76,6 +81,9 @@ services:
       # 587 is mapped even when SUBMISSION_ENABLE=false; Postfix listens only
       # when the variable is true — see docs/guide.md "Published ports".
       - "587:587"
+      # 25 is mapped even when INBOUND_RELAY_ENABLE=false; Postfix listens only
+      # when the variable is true.
+      - "25:25"
       - "127.0.0.1:8080:8080"
     # Hardening (spec 10 p.6). SelfPost's entrypoint still needs to run as
     # root very briefly to fix /data ownership and normalise permissions
@@ -85,7 +93,7 @@ services:
     # self-healing. What IS applied: no privilege escalation past what the
     # image already grants, and every Linux capability dropped except the
     # small set the root startup phase and Postfix/OpenDKIM genuinely need:
-    #   - NET_BIND_SERVICE — bind 465/587 (and 25 outbound) below 1024;
+    #   - NET_BIND_SERVICE — bind 465/587 and inbound 25 below 1024;
     #   - CHOWN            — entrypoint re-owns /data (bind mount) to `panel`;
     #   - FOWNER           — entrypoint then chmods those now panel-owned /data
     #                        dirs/files while still root (owner-check bypass);

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

deploy/traefik/docker-compose.traefik.yml

@@ -15,6 +15,7 @@ services:
     ports: !override
       - "465:465"
       - "587:587"
+      - "25:25"
     volumes: !override
       - ./data:/data
       - .:/selfpost-deploy:ro