Is there a better plan migrate e-mail systems?

Personally I wouldn't have done the forwarding thing, sucks that you weren't involved with the choosing of that migration phase.

I would have setup a gateway machine that mirrored all incoming email to the appropriate backend server, or even both machines while the migration was taking place.

Making one big move isn't a good idea, there's always something that is missed or that breaks that is unexpected, and when its effecting 150 people instead of 2, it'll be your head.

We last did this between 3 systems (Qmail, Exchange07(?), and Postfix) all with their own incoming and outgoing servers. We wanted to merge them all to Exchange 2010. We setup a postfix server to be the MX receiver for all 3 of those platforms. It received mail for everyone, scanned mail for spam, and then forwarded mail to the appropriate backend server based on a hash table. Clients still sent & received email without noticing any changes, and we slowly got to migrate our clients 1 set at a time. (clients didn't notice any changes until it was their day for upgrade anyway.)