Distribute mail based on sending domain?

[vr]

We have Exchange 2010 with a few domains and have run across the need 
to split outgoing mail direct to the Internet and also to smart hosts 
depending on their @domain.tld. Exchange 2010 does not support this "by 
design" so if Postfix does, is this functionality a relay? Looking at 
the BASIC_CONFIGURATION_README doesn't quite look like a match to my 
untrained eyes so any clues or configuration pointers are greatly 
appreciated.