Message routing loop may exist

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool queries the Active Directory® directory service to determine the value for the msExchAlternateServer attribute of each Simple Mail Transfer Protocol (SMTP) virtual server. The Exchange Server Best Practice Analyzer also determines whether the msExchNonAuthoritativeDomains attribute has been configured on any of the recipient policies in the organization. The Exchange Server Analyzer displays a warning if the following conditions exist:

  • Any instance of the msExchAlternateServer attribute has been set.

  • The msExchNonAuthoritativeDomains attribute has been set on any recipient policy.

The Exchange Server Analyzer displays a warning because the aforementioned configurations can cause message loops. Message loops can occur when different messaging systems are configured to operate together in a single organization. In this scenario, Exchange Server recipient policies and SMTP configurations are used to route and deliver messages for user groups that exist across heterogeneous messaging systems.

The msExchAlternateServer attribute is set when you specify an address in the Forward all mail with unresolved recipients to host field on an SMTP Virtual Server property page.

The msExchNonAuthoritativeDomains attribute is set when you clear the This Exchange Organization is responsible for all mail delivery to this address check box on the SMTP address properties page for a given recipient policy.

Therefore, if you specify an SMTP domain as non-authoritative by clearing the This Exchange Organization is responsible for all mail delivery to this address check box, and you have specified an address for unresolved recipients on any SMTP virtual server, the Exchange Server Analyzer displays a warning. However, it is important to note that the existence of this warning does not definitively indicate that a message loop is present in your organization.

If a non-authoritative domain exists and at least one server in the organization has msExchAlternateServer set, the potential for message routing loops exists. You should verify that the following conditions exist:

  • Authoritative domains exist for all domains in your organization.

  • Your SMTP virtual servers are configured so that authoritative domains are handling all unresolved messages.

Specifically, if you share a domain name with another organization, it is recommended that there is only one inbound path to that domain namespace. One mail server or one mail system should be authoritative for the domain, so looping does not occur. The authoritative server should be the last system on the inbound path.

It is not important which system is the initial receiver. It is not important if there are more than two initial receivers. It is important that only one system is the final destination.

To change the msExchAlternateServer attribute

  1. In Exchange System Manager, expand Servers, expand <Server Name>, expand Protocols, and then expand SMTP.

  2. Right-click your SMTP virtual server, and then click Properties.

  3. In the SMTP virtual server's Properties, click the Messages tab.

  4. In the Forward all messages with unresolved recipients to host box, type the FQDN of the server that will receive the unresolved mail. You may also enter the IP address of the server in square brackets ([ ]).

  5. Click OK.

To change the msExchNonAuthoritativeDomains attribute

  1. In Exchange System Manager, expand Recipients, and double-click <Recipient Policy Name>.

  2. On the E-Mail Addresses (Policy) tab, double-click an SMTP address.

  3. On the SMTP Address Properties page, a check in This Exchange Organization is responsible for all mail delivery to this address clears the msExchNonAuthoritativeDomains attribute. By clearing the check box, you set the msExchNonAuthoritativeDomains attribute with the SMTP address that is specified in the Address field.

For more information about sharing SMTP address spaces, see Microsoft Knowledge Base article 321721, "Sharing SMTP address spaces in Exchange 2000 Server and Exchange Server 2003" (

For more information about deployment scenarios for Internet connectivity, see "Deployment Scenarios for Internet Connectivity" in the Exchange Server 2003 Transport and Routing Guide (