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Q194589: XCON: Event ID 57 from the Exchange Message Transfer Agent

Article: Q194589
Product(s): Microsoft Exchange
Version(s): 4.0,5.0,5.5
Operating System(s): 
Keyword(s): 
Last Modified: 04-JAN-2002

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The information in this article applies to:

- Microsoft Exchange Server, versions 4.0, 5.0, 5.5 
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SUMMARY
=======

When a high volume of mail is queued up to other Exchange servers in the same
site, across a site connector, or across an X.400 connector, the Microsoft
Exchange Message Transfer Agent (MTA) may log the following event in the
Application Event Log:

  Event:57
  Source: MSExchangeMTA
  Category: X.400 Service
  Description:
  The limit on the number of associations allowed to and from entity
  /O=ORGANIZATION/OU=SITENAME/CN=CONFIGURATION/CN=SERVERS/CN=SERVERNAME/CN=MICROSOFT
  MTA has been reached. The limit is 9. [MTA MAIN BASE 1 43] (14)

Event ID 57 indicates that the number of messages in the queue exceeds the
maximum number of allowable associations between MTAs.

Mail may still flow sporadically or at a restricted rate, if the link and/or
remote MTA is still functional. MTAs of Exchange Server computers communicating
within a Site or across a site connector will use Remote Procedure Calls (RPCs).
We will refer to those MTAs as "MTAs communicating through RPC".

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

The text of Event ID 57 is as follows: The limit on the number of associations
allowed to and from entity <X.500 Distinguished Name referring to a remote
server> has been reached. The limit is 9. [MTA XFER-IN 16 34] (14)

Microsoft Exchange Server 4.0 and 5.0 handle associations differently than
Microsoft Exchange Server 5.5. Exchange Server 4.0 and 5.0 used 1 Control Block
(CB) per connector and had an unnecessarily complex algorithm to allocate which
association had use of the CB. Exchange Server 5.5 simplified the relationship
between associations and CB. There is now one association per CB. A control
block specifies how mail is processed within an association, or connection.

There can be multiple associations (or connections) between two Exchange Server
computers across an X.400 connector or between Exchange Server computers
communicating through RPC. Note that a connector is not the same thing as a
connection. The following discussion focuses primarily on the way Exchange
Server 5.5 handles associations between MTAs.

Exchange Server 5.5 allows a maximum of 10 associations between MTAs over an
X.400 connector or through RPC. This is further broken down to nine available
associations for low/normal priority mail and one association, which can be
opened for urgent mail.

A new association is opened up by default for every 50 queued messages to a
remote server. This setting is the Threshold value. It is found in one of two
different places, depending on how the servers are communicating to each other.

- If the MTAs are communicating across an X.400 connector, the Threshold value
  is found in the Exchange Administrator program if you click the X.400
  Connector's property pages, click the Override tab, click Association
  Parameters, and then click Threshold (msgs). The default value of 50 would
  allow 450 Normal and Low priority messages to flow (9 times 50) and 50 High
  priority messages to flow (1 times 50) before the maximum number of
  Associations are opened. Increasing this Threshold value to 100 would allow
  900 Normal and Low priority messages to flow and 100 High priority messages
  to flow.

- If the MTAs are communicating within a site, look in the Exchange
  Administrator program, click Configuration, click MTA Site Configuration,
  click the Messaging Defaults tab, click Association Parameters, and then
  click Threshold (msgs). If these servers are communicating across a site
  connector, the Threshold value should match at the MTA Site Configuration
  level.

Increasing the Threshold value will decrease the number of associations that have
to be opened to a remote MTA, and thus reduce or eliminate Event ID 57s.
However, if the bottleneck is the available bandwidth between servers, mail will
not necessarily move any faster.

Additional query words: association MTA Message Transfer Agent control block

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Keywords          :  
Technology        : kbExchangeSearch kbExchange500 kbExchange550 kbExchange400 kbZNotKeyword2
Version           : :4.0,5.0,5.5
Issue type        : kbinfo

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