Relay single Domain to relayhost?

[motokochan]

Unless you are doing something special with that domain, the MX should point to the most direct server to deliver to. By allowing relay to any address on that domain, you enable spam to any possible address, try to deliver to the actual mail server, then have to handle bounces - causing backscatter.

If you still want to act as MX, you should maintain a list of all valid addresses, and make sure they are in the mail map so you reject invalid addresses at the submission stage.