Hey Falko.
Thanks for pointing me towards the external domain. I selected external mail server for
domain.com (which is using gmail), and left internal mail server for
store.domain.com.
The users at store.domain.com have .forward files to forward to
user@domain.com
However the entries for
user@domain.com are still in /etc/postfix/local-host-names and /etc/postfix/virtualusertable even after restarting ISPConfig. Shouldn't they be removed?