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

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