Anyone know if there's a free version of this?
Try www.mxguarddog.com. It is free, and even though they are advertising it as a 'spam filtering service', it can acutally reflect incoming e-mails to an alternative port of the destination e-mail server.
I am using it to set up my own e-mail server at home. working great so far! (I just set up my server earlier today by the way, so I am keeping my fingers crossed that the mxGuardDog service stays free).