XCON: Event ID 57 from the Exchange Message Transfer Agent

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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.