Protecting Mail: Message Hygiene
Spam, viruses, and other junk is the part of managing a mail server that just plain sucks. There is so much junk out there that mail has to be treated far different than any other service. Special precautions need to be taken to reduce junk mail both within the server and before the junk mail gets to the server. Doing so will keep the server running optimally and keep the message queue from getting bogged down attempting to process mail that should have never been queued in the first place.
Spam Filtering via MX Records
One of the best ways to filter out spam and viruses is to just use a third-party service to do so. You can still enable various spam filters on the server itself (and you should). However, a third-party service will block out much of the junk mail before it comes into your network, freeing up your server to be far more effective in the tasks you actually want it to perform.
Most of the third-party spam filtration tools work by intercepting mail before it comes to your server and then delivering filtered mail to the server. These can operate in one of two ways. The first is as an add-on for your firewall, or gateway, appliance. Cisco, SonicWALL, Barracuda, and many others build solutions for these types of uses (you can easily build your own filter using SpamAssassin and put it in the demilitarized zone of your network, if you have one). The second way to filter mail before it gets to the server is to use a third-party service. Examples ...