I've been pouring over the Split Domain wiki, and some other posts on how to migrate from 2007 --> Zimbra 8.0

I am however not sure which course would be best, using Exchange as my primary or Zimbra. Also, we are using GFI Essentials and Mail Security on Microsoft SMTP in the DMZ, and I'm not sure if that would handle the split domain, or if I do it at the Exchange level.

Right now, mail routing is fairly straight forward:

ISP Relay <--> GFI SMTP Content Filter <--> Exchange 2007

GFI accepts mail for example.domain.com

Exchange is on example.otherdomain.com with example.domain.com as an alias.

I setup Zimbra 8.0 as zimbra.otherdomain.com with an alias of example.domain.com

I want to keep all user email addresses the same, hence the split domain.

So, my question is, do I need to get the GFI SMTP piece involved at all until I fully remove the Exchange server? Or can I just have mail routed from Exchange to Zimbra as we move departments then change GFI route mail just to the zimbra server when we are done?

I'm still a little confused on the best scenario to use. We don't have a lot of mailboxes (about 700), but they are rather large, and we cover over 35 departments --- so timing is crucial and the reason we what to do this in phases.