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Important This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:
(https://support.microsoft.com/kb/256986/ )Description of the Microsoft Windows registry
In a clustered Microsoft Exchange Server 2003 environment that also includes Microsoft Exchange Server 5.5 computers, the message transfer agent (MTA) service provides communication between the different versions of Exchange. However, the MTA service has a limit on the number of databases that are supported in this scenario. In a clustered environment, the MTA service supports a maximum of 50 databases.
To increase the number of databases that are supported by the MTA service to more than 50, you must modify the MTA registry entry for remote procedure call (RPC) threads.
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:
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.
(https://support.microsoft.com/kb/264075/ )Description of MTA tuning when Exchange 5.5 coexists with Exchange 2000 Server or with Exchange Server 2003