We've got an SBS2003 box that accepts mail from the outside fine except when it goes through Messagelabs. I have checked filtering on the server and cannot see anywhere why these emails would be blocked. We have tried from different domains that go through Messagelabs and they all return the following error:
From: MAILER-DAEMON@messagelabs.com [mailto:MAILER-DAEMON@messagelabs.com]
Sent: 12 December 2009 12:06
Subject: Expired Delivery Retry Notification (1 day ): Update
This is the mail delivery agent at messagelabs.com.
I was not able to deliver your message to the following addresses.
Connected to (wan ip of server) but connection died. (#4.4.2) Despite repeated attempts, this message could not be delivered.
--- Below this line is a copy of the message.
The server has AVG fro Exchange on the Protocols tree of the Exchange server but there's no console for it and it doesn't appear to be doing anything.