mailcow-dockerized/data/conf/postfix/config_templates/mysql_mbr_access_maps.cf.j2

user = {{ DBUSER }}
password = {{ DBPASS }}
hosts = unix:/var/run/mysqld/mysqld.sock
dbname = {{ DBNAME }}
query = SELECT CONCAT('FILTER smtp_via_transport_maps:', nexthop) as transport FROM transports
  WHERE '%s' REGEXP destination
    AND active='1'
    AND is_mx_based='1';