What version of IHSSE? Do you have maintenance coverage with your licensing? I had trouble with versions 1.4 and 1.4.5 and I found out about it using the message tracking in EXCH System Manager. The message from the sender [sometimes new messages, sometimes replies] would hit the server, be accepted for delivery and then go to the categorizer, yet never show in the inbox. I tried removing *ALL* my custom filtering rules and even raised the quarantine and delete thresholds. We can chat off line or keep things on board for others for more details.