I don't use Outlook, but can think of a couple obvious things to check. Is the PC in question continuously "on" and connected to the Internet, or is it off most of the time? Especially if it's often off, how often is Outlook configured to check in with its mail server and see what's there? If you manually order Outlook to fetch its mail (I presume that can be done), does it all show up immediately? Is the Internet connection very slow or unreliable?