Q182406: XFOR: Mail Looping When First Host Cannot Be Reached

Article: Q182406
Product(s): Microsoft Exchange
Version(s): WINDOWS:5.0
Operating System(s): 
Keyword(s): kbusage
Last Modified: 04-APR-1999

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The information in this article applies to:

- Microsoft Exchange Server, version 5.0 
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SYMPTOMS
========

When there are two servers listed in an Exchange record and the first host is
not available, the server then looks to itself to receive the messages. This is
not desirable due to the potential of looping.

STATUS
======

Microsoft has confirmed this to be a problem in Microsoft Exchange Server
version 5.0. This problem has been corrected in the latest U.S. Service Pack for
Microsoft Exchange Server version 5.0. For information about obtaining the
Service Pack, query on the following word in the Microsoft Knowledge Base
(without the spaces):

  S E R V P A C K

MORE INFORMATION
================

An SMTP host should deliver messages destined for a remote host to itself. After
you perform an Exchange record query, if one of the Exchange hosts is the host
doing the query, it must remove itself and all Exchange hosts with a weight
equal to its own from the list.

This is done for two reasons:

1. To prevent looping.

2. To avoid message backward flow (that is, messages moving to a less preferred
  Exchange host).

The Internet Mail Service handles this as long as it is running on a host with
one network interface card (NIC) that has one IP address.

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Keywords          : kbusage 
Technology        : kbExchangeSearch kbExchange500 kbZNotKeyword2
Version           : WINDOWS:5.0
Issue type        : kbbug
Solution Type     : kbfix

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