We are running Exchange 2003 Enterprise on Windows server 2003 SP1. Every once in a while a user complains about not receiving a message that was sent from outside our network, from an internet address. When I track the message in Exchange, I see that it arrived on our incoming mail smtp gateway server, but the process stopped at the Categorizer and the message was never sent to the user's mailbox server. There is a workaround that always works. If the sender of the message sends it to themself or someone else, and then the message gets forwarded to the person in our domain, then the user receives the message successfully. I assume this has something to do with message headers, but I'm wondering if there is anything we can do to correct the problem. This problem has happened with messages sent with and without attachments.
Thanks for any insight anyone can give me on this.