I'm not sure how you checked the user mail file to verify it is OK, but if you just checked disk space, permissions and did a simple more or cat to view the file this may be misleading. There could be a fualty control character or something in thisfile that is corrupting it and causing the problem. A simple test would be to
1)backup the file
2) cat /dev/null > file (this erases current mail waiting to be read but does not effect outgoing mail)
3) change permissions on this file so theuser owns it and has read/write access.
4) send an email to the user locally (as root or another account on the Linux server)
If he gets this the new mail file is good and something really was wrong with the old mail file.
If he doesn't getthis I have another question - how is the mail being delivered to his Outlook account. Is it via a parsing viewer directly to the Linux server or is it via a forwarder fromt he Linux server to an Outlook server. If it is via a parser you may have an incompatibility (did it ever work? and do other people use the same parser?). Maybe you could re-install the client parser on his PC. If it is via forwarder check /etc/mail/aliases for any references to his account that may be incorrect.
Hope this helps
