Sender domain based external routing

Hi,

We have the following routing scheme: One internal server in domino domain (A) and one domino domain (B) in DMZ acting as a relay.

We host 2 internet domains (@x.com and @y.com) and we would like to route mails originating from user1@x.com directly to internet (as it happens now) BUT to route user1@y.com through an external SMTP server (like a baracuda solution).

Any help would be greatly appreciated!

PS: We managed to route everything using a Foreign SMTP through the external solution, but have not been able to separate the traffic, based on the senders internet domain