Yeah, I followed the perfect debian install, then setup ISPConfig.
The problem is that I was running it under linux-vserver. Postfix couldn't send any mail to the domains with inbound MX records on the other IPs on the box as despite the fact it was only bound to a single IP I kept getting the "mail for example.com loops back to me" message. I know nothing about postfix and all the solutions said to add those domains to my list of local domains, which they're not. Thankfully I know a lot about Exim
I'll try and get a few minutes this week to write exim conf files for inclusion in a split Exim4 Debian config.