redhat - User account automatically filling up with dead.letter file

I have one user account on a server with about 400 accounts that is filling up automatically. The dead.letter file in the users home directory automatically grows until the account is full (about 10 - 40 Mb per day). The user is using Microsoft Outlook to send and receive mail.

What can be causing this and how can I avoid it from happening?

Right now I have an emergency cron-job to delete the file but I would like "real" solution.

Edit: The server version is Red Hat Enterprise Linux ES release 4 (Nahant Update 4)

Edit 2: It seems mainly spam and I see different mailer headings (from php to Outlook Express) and a frequent appearing header is USER_NAME@vsap.no.loop

Update: I have asked the hosting provider where I use that dedicated server to look into the problem as well, as it's their Control Panel that could be a cause of the problem.