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.
e.g.
mail.domain1.com should get routed to 10.10.10.1
mail.domain2.com should get routed to 10.10.10.2

Suggestions?