If you are running ICS and the outbound connection uses PPPoE (PPP over Ethernet), the clients may not be able to browse some sites or send e-mails that contain attachments.
PPPoE requires that the MTU (Maximum Transmission Unit) on all client computers be no greater than 1492. Depending on your setup, it may have to be lower.
To find the correct MTU size:
1. Locate the default gateway of the ICS server by running IPCONFIG.
2. Open a CMD prompt on a client and type:
PING -f -l <MTU size> <IP address of default gateway from step 1>
starting with an MTU size of 1464. If this works, add 28 to get the MTU size. If it fails, decrease the size and retry.