We faced a similar dilemma with an iMail migration to SmarterMail about a year ago. We setup different IP Addresses and Hostname bindings in SmarterMail for each iMail server that was migrated so that existing users could continue to login using just the first part of their email address as the User Name even though we were hosting hundreds of different domains on the same server. When we pointed DNS for those iMail domains to the new SmarterMail Server there was nothing that needed to be done client-side on previously configured devices or email clients, everything just worked! The only thing Tech Support needed to do was help webmail users who had the learning curve of a new interface. It was a huge time-saver for Tech Support to do it this way.

 

The only sticky widget for us is that we generally redirected all HTTP traffic for all of our domains to HTTPS on the Mail Server's primary Hostname. We had to make some creative URL Rewrites to exclude the old iMail domains from this redirect so that they could continue to login to webmail with just the first part of their email address as the username.