POP3 server tasks running 99 pct cpu

Hi,Since one week, our pop3 server task reaches 99 pct cpu while some different users connect with pop3 client like outlook.

At the same time a temporary file : c:\Documents and settings\user\Local settings\Temp\notesc839b6\st562678.tmp is created and its sizes increases until it reaches full disk size left (5 go) in a few minutes.

Any idea from a Domino Guru ? Many thanks