Description of MTA tuning when Exchange 5.5 coexists with Exchange 2000 Server or with Exchange Server 2003

Administrators may notice a slight performance degradation in Microsoft Exchange 2000 Server and Microsoft Exchange Server 2003 in comparison to Microsoft Exchange Server 5.5, particularly when the Microsoft Exchange Message Transfer Agent (MTA) Stacks service is used to send and receive mail. This article describes some of the performance tuning techniques recommended when you are deploying Exchange 2000 Server or later in a mixed environment.

Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk. Exchange 2000 Server and later do not include the Performance Optimizer (Perfwiz.exe). Instead, it relies on its various components to dynamically tune itself. In some cases, this does not yet hold true, as in the case of mailflow through the MTA.

There are two separate registry changes required, one for increasing the number of gateway threads for each Mailbox store, and one for increasing the maximum number of remote procedure calls (RPCs) in the MTA to accommodate the first increase. The following is an explanation of the registry keys that are mentioned in this article:

• The Gateway In Threads registry key defines the number of threads that are available for retrieving messages from the MTA process to the Store.exe process.
• The Gateway Out Threads registry key defines the number of threads that are available for sending messages from the Store.exe process to the MTA process.
• The Max RPC Calls Outstanding registry key defines the maximum number of remote procedure call (RPC) threads. This value limits the maximum number of RPC procedure calls that are guaranteed to be processed at the same time.

Changes for the Information Store

1. Start Registry Editor.
2. Locate the following key in the registry:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\<servername>\Private-<MDBguid>
3. On the Edit menu, point to New, click DWORD Value, and then add the following registry values:
   Value Name: Gateway In Threads
   Data Type: REG_DWORD
   Value: 3
   Value Name: Gateway Out Threads
   Data Type: REG_DWORD
   Value: 3
   Note You must add these registry values on all Mailbox stores (MDBs) for each Exchange 2000 or Exchange 2003 computer. That is, if there are two MDBs on this server, there will be two Private-MDBguid values in the registry.
4. Quit Registry Editor.

Important Do not implement the Information Store registry changes that are described in the previous method on clustered Exchange servers. This is because a clustered Exchange server runs a single MTA process that services all databases on all Exchange virtual servers in the cluster. If you increase the MTA gateway threads, there may be instability in the MTA process if the MTA is servicing more than 20 databases.

Changes for the MTA

1. Start Registry Editor.
2. Locate and then double-click the following value in the registry:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeMTA\Parameters\MAX RPC Calls Outstanding
3. In the Edit DWORD Value dialog box, click Decimal, type 128 in the Value data box, and then click OK.
4. Quit Registry Editor.