We have a setup where we have a bunch of mail we need to send to a domain that tends to back up our mail server. I put a transport rule in to send all mail to that domain to a relay to get it out of our main mail queue and let it deal with it and not disrupt the rest of the outgoing mail.
So far so good. But for redundancy and performance I would like to use several machines for that queue and have mail spread between the servers. I could use a load balancer, but is there a way to build a transport rule to say that all mail to aaa.com should be relayed through mail1.com, mail2.com, or mail3.com?