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Messages Queue on an Edge Transport Server with 500 5.1.1 Unrecognized Command Error

 

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:

noteNote:
The SMTP or SMTPS filter should be disabled only on those SMTP transport rules that are published between the Hub Transport server and the Edge Transport server for mail communication.
noteNote:
The SMTP filter is applied to the Mail Server Publishing Rule created on ISA Server 2006 at the organization level and cannot be removed unless the SMTP filter is disabled at the organization level. For more information, see How to Disable the SMTP Filter or SMTPS Filter on ISA Server 2006 at the Organization Level.

For more information about how to configure an ISA Server 2006 firewall between your Hub Transport servers and Edge Transport servers, see the following topics:

 
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