I am beginning to brainstorm the possibility of moving my current webmail solution on a shared general purpose webserver from Horde to open source Zimbra, and I'm wondering if you folks could answer a few questions for me...

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?
I'm currently using a Postfix / Dovecot / Horde solution. How hard would it be to migrate my users? I'd prefer not to have to change *anything* about my setup, other than the webmail interface (removing Horde, implementing Zimbra).

How much memory, cpu load, etc.... would you estimate would be needed for about 10 low-traffic (and low-email traffic) domains?