How to increase the number of databases that are supported by the MTA service when Exchange Server 5.5 coexists with a server cluster that is running Exchange Server 2003

When you run a mixed environment of Exchange 2003 computers and Exchange Server 5.5 computers in your organization, you must use the MTA service to communicate between the different versions of Exchange.

In a clustered Exchange 2003 environment, when you must run Exchange Server 5.5 in the same routing group as Exchange 2003, the use of the MTA service imposes restrictions on the maximum number of databases that are supported in the server cluster. This article describes how to increase the maximum number of databases that can be supported in the server cluster. To increase the maximum number of databases that can be supported in the server cluster, you must modify the MTA registry entry for RPC threads.

In a clustered Exchange 2003 environment that has Exchange Server 5.5 computers in the same routing group, the total number of databases that are supported is limited. If you exceed the limit, you experience the following:

• The following event is logged in the Application log to indicate that the MTA service could not create an association with one or more databases:

  Event Type: Warning
  Event Source: MSExchangeMTA
  Event Category: Resource
  Event ID: 3121
  Date: date
  Time: time
  User: N/A
  Computer: ServerName
  Description: A resource limit has been reached when attempting to open an association. Number of associations configured: 501 [XAPI MAIN BASE 71 91] (14)

• Messages to users who are hosted on databases with which the MTA service could not create an association build up in the message queues.

In this situation, you can modify a registry entry to increase the maximum number of supported databases to more than 50. To do this, follow these steps.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Note You must follow these steps on the Exchange 2003 computer that is running the MTA service. If you do not have any Exchange Server 5.5 computers in the particular routing group, you can disable the MTA service on the server cluster. If you disable the MTA service on the server cluster, you no longer experience the limit of 50 databases.

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeMTA\Parameters
3. In the right pane, right-click Max RPC Calls Outstanding, and then click Modify.
4. In the Value data box, type 00000080 (hexadecimal) or 128 (decimal), and then click OK. If this value is not changed to 128, it will not allow for the 50 databases.
   
   Important Do not change the Gateway In Threads registry value or the Gateway Out Threads registry value in this scenario. In a clustered environment, changing these registry values may cause the MTA process to become unstable.

For more information about how to tune the MTA settings in a non-clustered environment, click the following article number to view the article in the Microsoft Knowledge Base: