Before I start, I am researching a more detailed answer however below are the basics.
Programs:
Mail Security for Domino
Mail Security for Exchange
Brightmail Message Filter
Brightmail Gateway
Exchange and domino are both installed on the mail server and are designed to scan the message stores. Symantec Brightmail Message Filter is designed to be install on a Windows or Linux box and is an in-line scanner. It can also be installed on the mail server however it is normally placed "up Stream" of the mail server. Brightmail Gateway is a stand along appliance (Hardware purchased through symantec or installed into VmWare ESXI/ESX). It is designed to be installed into the mail flow most commonly in your DMZ. It has it's own OS that requires little configuration and even less interaction after installed.
Let me know if this helps.
Thanks,
John