About Symantec Message Filter deployment at the gateway layer
Some organizations prefer to have secure gateways with no other active services. In these environments, all other services including antispam services function behind the first gateway layer.
In this deployment configuration, Symantec Message Filter resides at the outermost gateway layer. This layer contains the gateway MTA, which processes inbound mail and relays it to other relay layers or to the user-facing message store layer.