Disappearing Mail

We have an exchange server (5.5) running, there are no restrictions set for this particular user. He is the ONLY one with this problem, which is, mail messages are disappearing after about 6 or 7 days. I’ve checked his view settings and cleared out any possible filters that may have been set, and made sure the delivery option goes to the correct mailbox. I cannot find anything helpful on microsoft knowledgebase…….can someone help…Thanks in advance.