It sits on top of Postfix
Does Zimbra have its own MTA, or does it sit on top of something like Postfix?
Can you have multiple domains in a single Zimbra installation?
Depends on the size of their mail boxes and the number of users. I've tried IMAPSync on my home server and didn't really think it worked well. I'm currently working on our move over, at my place of employment, we only have pop3, so I'm manually zipping up all the mail boxes, linking into the new account with Thunderbird and pushing mail out to the new setup via IMAP.
I'm currently using a Postfix / Dovecot / Horde solution. How hard would it be to migrate my users?
It's not going to be as easy as that for the Admin.
I'd prefer not to have to change *anything* about my setup, other than the webmail interface (removing Horde, implementing Zimbra).
The number of domains isn't the issue, it's the number of mail boxes you'll have to deal with.
How much memory, cpu load, etc.... would you estimate would be needed for about 10 low-traffic (and low-email traffic) domains?
We're running the NE version with around 18 domains on a Dell PowerEdge 1850 2U, 600GB drive space (SCSI 15000RPM) and 10GB memory. For 196 mail boxes at this time, I'm at 42% space. We average in the lower 25% CPU usage.
My home server with 3 domain and around 15 accounts, I run under VirtualBox with 2 CPUS and 4GB memory. I think it's pretty over kill though.