One server that had been running faithfully for years stopped sending/receiving emails even locally, and would not do so until virus scanning was disabled. Server was updated, same result. However, I should note that this machine is a 500 MHz Athlon with 512 megs of RAM so I am thinking the hardware just isn't up to the task. I've ordered a replacement machine (2 GHz Xeon, 2 gigs RAM) and will do the usual Affa replacement and am hoping that'll take care of the problem.