Recive old mails

We are using Mithi mail server on Linux system and Microsoft outlook as mail client on Win2000, one of my user receiving old mails (mail older than 3-4 months ) in his inbox. Outlook is configured with ‘leave a copy of message on server’ only option. He is able to see his old mail(no new pst file is created).
What is the problem , why he is receiving old mails.