Log on as Global Administrator, open up the mailbox in a different version of Outlook if possible Outlook XP. Does his sent items crash? If you are able to view the sent items have a look through for any emails with file attachments, save the attachments to the hard drive then delete the emails. Also delete as much as possible etc. Also delete and save attachments from all other folders including the inbox. Make sure if you have mailbox limits that the user hasn't exceeded the limit.
I get this problem a lot with users, and they are normally using old machines running Outlook 97. If possible upgrade outlook to outlook xp and you will probably find the problem goes away.