Gateways

The purpose of a Mail Gateway is to create a mail server that accepts mail for the internal mail server, thus protecting the internal mail server from outside attacks. Mail Gateways are typically on a separate network from the internal network. The gateway will show up in the DNS records as the final destination for mail even though it is transferred into the internal mail server.
One thing that the gateway can do for you is to do the spam filtering and virus filtering before it gets into your internal network . One important advantage to this set up is that you only need to manage filters on one mail server as it could do all the heavy work before relaying to your other mail server.

The implementation of the gateway is fairly simple. The basic set up only takes a few lines of changes. The more advanced set up requires additional time but is probably a good idea as it will provide enhanced security for the gateway which will certainly be under greater attack. The other issue to be aware of is that the more requirements you have for the gateway in terms of scanning for spam and anti-virus will always require more RAM and CPU resources as well. However, this requirement will then lead to less required RAM and CPU resources on the internal mail server.