MTA TCP/IP control blocks parameter is non-default

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2007-11-14

The Microsoft® Exchange Server Analyzer Tool reads the following registry entry to determine its current value:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeMTA\Parameters\TCP/IP control blocks

If the Exchange Server Analyzer finds the value for TCP/IP control blocks present and set to anything other than 20, a non-default configuration message is displayed.

The TCP/IP control blocks value is the maximum number of concurrent RFC1006 (TCP/IP) connections that are supported. This setting controls the number of buffers available for X.400 connections. If this message transfer agent (MTA) hosts more than one X.400 connector, the TCP/IP control blocks value should be adjusted. There should be 10 control blocks for each X.400 connector being hosted, and one control block to service incoming connections. Therefore, for a system with three X.400 connections, the appropriate value would be 33.


This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore the registry if a problem occurs. For information about how to restore the registry, view the "Restore the Registry" Help topic in Regedit.exe or Regedt32.exe.

To configure the TCP/IP control blocks value

  1. Open a registry editor, such as Regedit.exe or Regedt32.exe.

  2. Navigate to: HKLM\System\CurrentControlSet\Services\MSExchangeMTA\Parameters

  3. In the right pane, locate and double-click the TCP/IP control blocks value.

  4. In the Value data field, enter the value appropriate for this MTA, and then click OK.

  5. Close the registry editor and then restart the Microsoft Exchange MTA Stacks service for the change to take effect.

Before you edit the registry, and for information about how to edit the registry, see the Microsoft Knowledge Base article 256986, "Description of the Microsoft Windows Registry" (