I have setup message screener on my isa 2004 sp1 on a W2K3 sp1 server with smtp vertual server on the same computer, exchange 5.5 is running on a nt4.0 sp6a computer on my internal network. The problem i am having is some messages are getting ndr's that say Unknown Recipient, but only from certain senders. If i send a test message to the same recipient from my hotmail account the message goes through fine. No domains are blocked, and if i disable the message screener everything returns to normal. the only thing i have blocked at this time is keywords which are set to forward to my account if triggered. This problem happens when i turn off the smtp filter and use the isa as a vurtual smtp server to relay to the exchange 5.5.