After a lot of testing, I now want to migrate my users over to Zimbra. This isn't a technical problem, more a logistical one.
I have set-up my old e-mail server to forward any mail for me to my new Zimbra server (using an alias). This works great.
I would like to set-up Zimbra, to forward all email to unknown users back to the old server. This way I can slowly migrate users, as and when they and I have time.