Email automatically deleted after few minutes in Outlook
Solution 1:
Have you checked that he doesn't have a POP3 client set up somewhere that is connecting to the server, retrieving the e-mails, and deleting them?
Perhaps he has a mail client set up at home so he can get messages there too, and doesn't realize this will delete the messages off the server.
Solution 2:
The interval sounds like a external IMAP / POP client is checking in, getting the messages and then deleting them from the server. Is he using a smartphone, blackberry, email client at home that may be doing this?