Howdy. I'm sort of beating my head off of an issue here, and I'd kind of appreciate some feedback.
We're going to be migrating roughly 500 accounts from an old server running @Mail (built on Exim) to Zimbra, probably 10 accounts at a time as various regional offices make the switch. The obvious downside is that both servers have to be working throughout the entire migration process.
Zimbra is currently configured to be email.ourdomain.com. The original box is mail.ourdomain.com, and self-identifies as ourdomain.com.
Our gateway MTA (Mailgate, for simplicity) is running Postfix, with virtual_alias_maps and relay_recipients configured to forward all valid accounts to @Mail, which in turn forwards the few beta test users' mail to Zimbra. Obviously this solution doesn't scale for 500 users!
I'm looking for any ideas anyone may have had / seen done for this sort of scenario. Some users will ultimately be remaining on @Mail after the migration is complete (automated accounts that don't need collaboration features, will ultimately be rolled over into a Courier instance), so any final solution will need to take this into account as well.
My apologies if I've left out anything important; please let me know if there's anything I can add to make this more clear.
Thank you in advance.