How to route x.x@y.com based on x.x

We have a number of separately maintained and managed Exchange, GroupWise, and Lotus email systems. They each have a separate domain (?@individual-domain.com) currently. The desire is to change all email addresses to x.x@single-domain.com but maintain the separate email systems. Does anyone know of a product or configuration that can route email based on the local address or x.x portion of the email address? If you have used such a product or configuration, what kind of administrative overhead in staff time does it require?