If you think the mailbox might be corrupt what you can try to do is export all the mail from this user into a .pst file (personal folder), verify all the data is in the pst file that was in the mail account.
Then kill off the account on exchange (** You might want to also note the user's SMTP mail address or an extras before you blow away the account), recreate the mail account and import all the data back into the new account on Exchange.
This will show you if there was a problem with the mailbox or not and also give you a clean slate but allow all the user's data to be intact.
Let me know if that works for you.