Transition from plain sendmail

[Mistoffeles]

So I have this decrepit old FC4 box and I am replacing it with separate email and web servers, and I need to support a couple hundred users.

The first domain I'll be putting on the server is unrelated to this domain, it will be just a few employees using this.

The second domain, we'll call it pretend.com, is set up with an MX record of

@ MX 10 mail.pretend.com

and an A record of

mail A 10.10.10.10 (yeah, not really)

I want all new incoming mail to go to the new server, and all outgoing mail to come from the new server, but I also want any old mail still rattling around on the old server to be sucked up into their mail clients too (the aimbra webmail interface will be available optionally).

Is there a way to set this up in DNS, or do I have to mess around with having Zimbra suck their old email off the the old server and into the new one, or go through a migration process than I know I won't enjoy?