Microsoft introduced TCP/IP Auto tuning with Vista and Windows 2008 server. Its also a part of Windows 7 and 2008 R2. This feature resizes IP packets which can improve network performance. Older network devices, however, like Cisco PIX firewalls do not support Auto tuning and can cause connection issues.
I had a client that was getting email undeliverable errors from a certain domain while successfully sending to various others. After investigating OBL listings and various other possible culprits, the tech for the problematic domain suggested looking at this. My client has an Exchange 2007 environment installed on Windows 2008 SP2 servers. Sure enough after disabling this feature on all the hub transport servers, the SMTP traffic was no longer timing out to this domain and mail flow was established.
To disable TCP/IP Auto Tuning, run the following from an elevated command prompt:
| netsh interface tcp set global autotuning=disabled |
For more information on TCP/IP Auto Tuning, Visit:
http://blogs.msdn.com/b/wndp/archive/2006/05/05/winhec-blog-tcpip-2.aspx