Mail flow to certain domains does not work when you run Exchange Server 2007 on a Windows Server 2008-based computer

On a Windows Server 2008-Based server that is running Microsoft Exchange Server 2007, mail flow to certain domains does not work. For example, you may be unable to send e-mail messages to several domains.

Note If you use the Telnet utility, and you use port 25 to telnet to the domains to which you have problems sending e-mail messages, you receive a blank screen. The blank screen does not have the expected banner. When you use the ehlo domain.com command, you see the Simple Mail Transfer Protocol (SMTP) commands on the screen. However, for the same domains, if you telnet to port 25 from a Windows Server 2003-based server, you receive the banner on the screen as expected.

This problem occurs because routers do not support the TCP autotuning settings in Windows Server 2008. This article helps you work around this problem.

To work around this problem, disable autotuning. To have us disable autotuning for you, go to the �Fix it for me� section. If you would rather disable autotuning yourself, go to the �Let me fix it myself� section.

Fix it for me

To disable autotuning automatically, click the Fix this problem link. Then click Run in the File Download dialog box, and follow the steps in this wizard. Fix this problem
Microsoft Fix it 50056
Note this wizard may be in English only; however, the automatic fix also works for other language versions of Windows.

Note If you are not on the computer that has the problem, you can save the automatic fix to a flash drive or to a CD, and then you can run it on the computer that has the problem.

Now go to the "Did this fix the problem?" section.

Let me fix it myself

To disable autotuning, follow these steps:

1. Click Start, type the following in the Start Search box, and then press ENTER:
   runas /user:Local_Computer_Name\administrator cmd
2. When you are prompted for the administrator account password, type the correct password, and then press ENTER.
3. At the command prompt, type the following command, and then press ENTER:
   netsh interface tcp set global autotuninglevel=disabled
   This command disables the Receive Window Auto-Tuning feature.
4. Exit the Command Prompt window.
5. Restart the computer.

Now go to the "Did this fix the problem?" section.

Note If you want to enable autotuning, open a command prompt, type the following command, and then press ENTER:

Did this fix the problem?

Check whether the problem is fixed. If the problem is fixed, you are finished with this article. If the problem is not fixed, you can contact support.

By default, TcpWindowScaling is turned on in Windows Vista and in Windows Server 2008. Some routers cannot handle scaling features. Most of these routers are small home-based routers. We recommend that routers are upgraded so that they work correctly with TcpWindowScaling. Window scaling is a TCP feature.

The following autotuning settings are available if a router supports TcpWindowScaling:

• Disabled: Fix the receive window at its default value.
• Highly Restricted: Enable the receive window to grow beyond its default value, but do so very conservatively.
• Restricted: Enable the receive window to grow beyond its default value, but limit such growth in some scenarios.
• Normal: Enable the receive window to grow to accommodate most scenarios.
• Experimental: Enable the receive window to grow to accommodate extreme scenarios.
  
  Note The Experimental setting can significantly decrease performance in typical scenarios. This setting should be used only for research purposes.

For more information about the TCP/IP stack that includes Receive Window Auto Tuning, visit the following Microsoft Web site: For more information about Windows TCP implementation features, visit the following Microsoft Web site: For more information about TCP receive window size and about window scaling, visit the following Microsoft Web site: For more information, see Request for Comments (RFCs) 1323, "TCP Extensions for High Performance." To see RFC 1323, visit the following Internet Society Web site: Microsoft provides third-party contact information to help you find technical support. This contact information may change without notice. Microsoft does not guarantee the accuracy of this third-party contact information.