A Windows XP SP1 and higher machine with Outlook 2002, ISA Firewall Client, and Exchange 2000. The client has Outlook open. A new e-mail comes in to that persons box from either in-house or outside. The message does not show up in the users inbox and they are not notified of a new message. The user clicks out of the inbox into say the calendar and back in. That is when all new mail arrives. Send and Recieve has no effect.
After hours and hours of troubleshooting and finally a MS Support incident (that I was able to get refunded) we figured out the Firewall client (irrelevant whether it is enabled or disabled) conflicts with Windows XP SP1 and higher, Exchange Server, and Outlook XP and causes this problem. Uninstalling the FW Client solved the issue.
Also, this was not an isolated incident.. Observed it with Outlook 2000, 2003, and 2003 on a mix of Exchange 2000 and 2003 on a number of different networks. Common thread is Windows XP SP1 and SP1a with the Firewall Client.