The 7200 is a relatively recent computer, so yes, it can run 24/7 without any problems. I have a 9600 which runs as a server continuously. It's excellent hardware.

Too bad you don't something with a 604(e). If you did, OS X would easily be best for mail serving. Of course there are various email server programs for System 7 and 8, but they're obviously older and most likely commercial.