I thought I will put this info on this here, so that everytime I answer a question, don’t have to type it all and just link to here;

Basically one of the biggest problems encountered with ExchangeServer and Outlook client when they are connected over a IPSec VPN, it doesn’t work very well. As a solution, this is what I’ve always suggested and proved to be of use too.

1. When you connect through VPN, first find out what is the best MTU size for you to talk to your corporate. How you can do this is simple; Connect to VPN and then,

ping -l 1400 -f <insideipofServer>

what we are doing here is to set the mtu to 1400 and also set ‘don’t fragment’. So if it is possible to send the packet without fragmenting then it would go through otherwise you’ll get a reply saying ‘don’t fragment bit set and so cannot proceed’. You are in-effect finding the Path MTU.

Then slow start reducing the 1400 to 1350, 1300 etc and see when you can ping without any problems and that should be your MTU.

2. Also make sure you add the servername to ip resolution in your hosts file.

That should take care of your problem in most of the scenarios. Now how to change the MTU size on the network adaptor?

The MTU for Windows 2000/XP/2003 network interfaces can be configured here:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{Interface GUID}\MTU