I have a client that has several offices in different physical locations, each with their own hosted email servers in a variety of flavors (postfix, exchange, unix mail). Each location has a different domain. They are closing several locations and want to put all the servers into a single existing data center where they have a single internet connection (1 IP address). They prefer not to consolidate the mail servers at this time.

I am trying to figure out the best way to route the incoming mail traffic (pop and smtp) to the appropriate server.
mail.domain1.com should get routed to
mail.domain2.com should get routed to