[Postfix] Allow to relay only non-local mailboxes

data/Dockerfiles/postfix/postfix.sh

@@ -38,6 +38,23 @@ for cert_dir in /etc/ssl/mail/*/ ; do
 done
 postmap -F hash:/opt/postfix/conf/sni.map;
 
+cat <<EOF > /opt/postfix/conf/sql/mysql_relay_ne.cf
+# Autogenerated by mailcow
+user = ${DBUSER}
+password = ${DBPASS}
+hosts = unix:/var/run/mysqld/mysqld.sock
+dbname = ${DBNAME}
+query = SELECT IF(EXISTS(SELECT address, domain FROM alias
+      WHERE address = '%s'
+        AND domain IN (
+          SELECT domain FROM domain
+            WHERE backupmx = '1'
+              AND relay_all_recipients = '1'
+              AND relay_unknown_only = '1')
+
+      ), 'lmtp:inet:dovecot:24', NULL) AS 'transport'
+EOF
+
 cat <<EOF > /opt/postfix/conf/sql/mysql_relay_recipient_maps.cf
 # Autogenerated by mailcow
 user = ${DBUSER}