Exchange 2007 on Windows 2008 does not deliver mail to some domains

Exchange 2007 running on a Windows Server 2008 server does not properly deliver mail to certain domains.

Further testing finds that attempting to telnet to port 25 from the server to the remote domain provides a blank screen instead of providing the expected header.

Microsoft reports that this problem occurs because routers do not support the TCP autotuning settings in Windows Server 2008.

Resolution:
The resolution to this issue is to disable AutoTuning

Click Start, type the following in the Start Search box, and then press ENTER:
runas /user:Local_Computer_Name\administrator cmd

When you are prompted for the administrator account password, type the correct password, and then press ENTER.

Open a 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.

Exit the Command Prompt window.

Restart the computer.

source: http://support.microsoft.com/kb/951291/