I have a client who uses Outlook Express to retrieve their e-mail most of the time. The problem occurs when they are away from their main PC and attempt to retrieve messages via webmail. Incoming messages do not appear in any of their inboxes. Not even new ones as they are sent.
I have tested this phenomenon thusly
Close all e-mail clients associated with the account (clients local Outlook client)
Send a test e-mail to the account
Nothing appears in the inbox, at all, not even after an hour of banging on the F5 button. This step has been tested with each webmail client on the server
Then after frustration has gotten the best of me, I launch the local Outlook mail client and immediately the test message appears in the inbox.

Thanks for any input!

Mike