Appointment and Meeting Request Issues

We are having an issue with a user that is currently running Outlook 2003.
The issue is that when someone sends him a Meeting or Appointment Request, he
does not get the email invite to Accept or Decline. 2 other people are
getting the invites, even though they are not on the request. About 5
minutes later the user that had the invite sent to, has the meeting show up
on his calender, even though he never gets the email invite. This has
started to cause problems, as he is unaware of meetings because he is not
getting the invites. He is running Outlook 2003, and Windows XP. We are
running Exchange 2003 SP2. Does anyone have any suggestions???