mail server probs


I am using on NT 4.0 SP3, with over 10000 accounts and am
running into some trouble. Its running on a dual PPro200 512k cache per
chip, 2meg cache on board, 512M RAM, RAID-5 10meg buffer disk totaling 32gig.
I've got this problem where the POP3 dies every 10 minutes or so. There is
usually 50 SMTP connections, and 10 POP3's when it dies. CPU and Disk I/O
are relatively low. The service needs to be stop/started for it to work
again (for next 10 mins). I've checked for malicious activities and have
pretty much ruled them out (things like socket flooders). Its pretty much
got me stumped. Any suggestions?