Just a point to note, to get canonical or virtual addresses working in postfix with a simple @domain.com points to @domain.co.uk is a real pain in the bottom. The best guide I found was:
But this still relies on pointing to a USER. Not fun when there are 10s or 100s of mailboxes needing mapping. Canonical mapping didn't seem to work, despite reading lots of documentation on it (a nice howto like the link above wouldn't go a-miss).
I only use ISPConfig for web/db hosting but one client couldn't connect to my hMailserver host, and they just happened to have tons of mailboxes (lucky me). As soon as a way can be implemented to get ISPConfig using virtual email domains, I think it would be well used by many.