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>

cmd/panel/envdoc_test.go

@@ -15,6 +15,11 @@ var documentedPublic = []string{
 	"PANEL_SESSION_IDLE_DAYS",
 	"SELFPOST_DNS_RESOLVERS",
 	"TRUSTED_PROXY_CIDR",
+	"INBOUND_RELAY_ENABLE",
+	"INBOUND_ANTISPAM_MILTER",
+	"INBOUND_ANTISPAM_MILTER_ACTION",
+	"INBOUND_RATE_LIMIT_MESSAGES_PER_IP",
+	"INBOUND_MESSAGE_SIZE_LIMIT",
 }
 
 // documentedInternal matches architecture.md § Configuration "Internal env vars".
@@ -34,6 +39,10 @@ var documentedInternal = []string{
 	"POSTFIX_DIR",
 	"POSTFIX_SENDER_LOGIN_MAPS",
 	"POSTFIX_QUEUE_DIR",
+	"POSTFIX_RELAY_DOMAINS",
+	"POSTFIX_TRANSPORT_MAPS",
+	"POSTFIX_RELAY_RECIPIENTS",
+	"POSTFIX_TLS_POLICY_MAPS",
 	"SELFPOST_DEPLOY_ROOT",
 	"MILTER_CONNECT_TIMEOUT",
 	"MILTER_COMMAND_TIMEOUT",
@@ -73,6 +82,7 @@ var loadConfigKeys = []string{
 	"SASL_REALM",
 	"POSTFIX_DIR",
 	"SELFPOST_DEPLOY_ROOT",
+	"INBOUND_RELAY_ENABLE",
 }
 
 // buildScriptKeys is every ${VAR:-…} / os.Getenv used in build/*.sh and entrypoint.sh
@@ -90,6 +100,15 @@ var buildScriptKeys = []string{
 	"POSTFIX_QUEUE_DIR",
 	"SASL_DB_PATH",
 	"SUBMISSION_ENABLE",
+	"INBOUND_RELAY_ENABLE",
+	"INBOUND_ANTISPAM_MILTER",
+	"INBOUND_ANTISPAM_MILTER_ACTION",
+	"INBOUND_RATE_LIMIT_MESSAGES_PER_IP",
+	"INBOUND_MESSAGE_SIZE_LIMIT",
+	"POSTFIX_RELAY_DOMAINS",
+	"POSTFIX_TRANSPORT_MAPS",
+	"POSTFIX_RELAY_RECIPIENTS",
+	"POSTFIX_TLS_POLICY_MAPS",
 	"MILTER_CONNECT_TIMEOUT",
 	"MILTER_COMMAND_TIMEOUT",
 	"MILTER_CONTENT_TIMEOUT",