If their mail host is not the same server you are communicating with otherwise, you can set up a host route to their mail server pointing at the internet connection, and make certain this gets into your routing protocol in the rest of your domain (but doesn't leak outside your domain, into the internet, for instance). You could do this with a static/redistribute static (don't forget the default metric), or a number of other ways, but the principle is the same in each case.
Russ