Maximum Segment Size (MSS) value advertised by a Server or a System is the preffered size of the segment that it can receive. For an standard packet this is equivalent to the Maximum Transmission unit (MTU) – 40bytes (standard TCP/IP overhead of 40 bytes [20+20]). This is the value that the server advertises and not what we can transmit.
If the MTU is 1500 bytes then the MSS will be 1460 bytes.
Recently we had this problem with this problem with an Exchange 2003 server in the HQ and Outlook Clients in a particular branch office. The Branch office connects into the HQ through a site to site IPSec VPN using Juniper Netscreen SSG20 firewalls on either end of tunnels.
The Problem
The Outlook clients would connect OK but suddenly loose connection to the Exchange server and never connect back. The Outlook Client status will say "Disconnected". The client PCs will however be able to ping the server and network connections look OK. This happened in random times and sometimes when sending large emails.