I've just installed Zimbra 4.5.7 under Ubuntu 6.06. Our company is migrating from sendmail running on FC3. Here's the dilema. I have two MX records:

example.com. MX 10 mail1.example.com.
example.com. MX 10 mail2.example.com.

mail1 is the sendmail system, mail2 is the shiny new Zimbra system.

We're planning a phased migration and during that time, we will have users on both systems under the same domain. Both mail1 and mail2 need to be aware of all of the users of example.com regardless of whether their mailbox resides on mail1 or mail2 so that delivery of messages (from local or remote sources) should be possible for any user in the example.com domain.

What do I need to configure on these two servers to make them aware of each other in domain example.com?