I've ran into an unexpected problem relaying emails sent to a domain to another host. In ISPConfig I've added the mail domain and set a relayhost for it expecting all emails sent to the domain would be relayed to the set host. Except now I see errors in the logs saying the email address doesn't exist. So it looks like emails are not relayed but delivered local. Testing the query in mysql-virtual_sender-relayhost.cf (which is set in Postfix' main.cf for sender_dependent_relayhost_maps) does give me the relayhost I've set. If I delete the mail domain in ISPConfig and create a map file for sender_dependent_relayhost_maps by hand it does work as expected.