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, ......