I currently have my old server still chugging away in the basement running postfix and delivering mail to several users. And now, I am planning on installing a new server to take over for the older one.

What I would like to do is transition the new server in slowly, basically accepting mail delivery for the few users set up on the new machine, then passing any users it doesn't see as local onto the old server for delivery there. Anyone know an easy way to accomplish this? relay_domains seems to be the right idea, if it would somehow attempt local delivery first. Using the virtual file seems to be another way, but would require every possible e-mail address to be listed in there, and I do not have every on-the-fly address my users have made.