Would it be reasonable to be able to have ISPConfig 3 control most hosting systems, but have all e-mail related stuff handled by Zimbra?
I know this would mean IT Admins would have two separate systems to administer, but it should be able to work. shouldn't it?
Just specify the mx record to go to zimbra, and disable all the e-mail server stuff in ispconfig. This way ispconfig controls the dns, websites, etc and zimbra controls the e-mail.