The problem is that my coworker has a desktop and a laptop (both Mac), both with the same mail account set up. He has set them both to leave a copy of the email on the server and delete after 15 days. The problem is that it doesn't seem to actually delete them on the server, and, periodically it will just re-download all the old messages even though that computer already has them all.
What could the problem be? Is there any way to check if there is an error with his setup or software?

The reason I am posting this instead of him is because I am responsible for the email hosting and he thinks it is a problem at my end. I have checked it with Outlook Express on my PC and everything seems to be working fine. I have also checked out the server for any errors with his mailbox and I can't find anything wrong.

Any help is greatly appreciated!