Applies to: Exchange Server 2007 SP1, Exchange Server 2007
Topic Last Modified: 2007-09-25

This topic explains how to resolve the problem of inbound Internet e-mail messages that are stuck in queues on the Edge Transport server when the Edge Transport server tries to send mail through Microsoft Internet Security and Acceleration (ISA) Server 2006, with Simple Mail Transfer Protocol (SMTP) filtering enabled, to internal Hub Transport servers.

When this problem occurs, the following error typically appears in the Last Error column in the Queue Viewer in Microsoft Exchange Server 2007:

• 500 5.1.1 Unrecognized Command
  
  

In this situation, ISA Server 2006 is installed as a firewall between an Edge Transport server and a Hub Transport server on the perimeter network of an Exchange 2007 organization. Inbound Internet mail messages to the Hub Transport server are stuck in queues on the Edge Transport server. This happens because SMTP verb commands issued by the Edge Transport server are not understood and are filtered by ISA Server 2006. This results in a 500 5.1.1 Unrecognized Command error on the Edge Transport server.

You can clear the mail queues on the Edge Transport server in one of the following ways:

• Add the Exchange SMTP verb commands to the SMTP add-in filter on ISA Server 2006. For more information, seeHow to Add SMTP Verb Commands to ISA Server 2006.
  
  
• Disable the SMTP or Secure SMTP (SMTPS) traffic filter on the Mail Server Publishing Rule for the Hub Transport server on ISA Server 2006. For more information, see How to Disable the SMTP Filter or SMTPS Filter on Firewall Policy Rule on ISA Server 2006.
  
  

For More Information

To ensure that you are reading the most up-to-date information and to find additional Exchange Server 2007 documentation, visit the Exchange Server TechCenter.

---

© 2008 Microsoft Corporation. All rights reserved. Legal Information