The amount of e-mail spam traffic worldwide has been increasing exponentially every year. There are number of tools to filter some of this unwanted e-mail traffic. The tools are mainly split between protection at the actual mail server and protection at the computer level. Several companies like Symantec, TrendMicro, and others provide protection at the computer level. Other services like Postini and Brightmail filter mail before it even gets to the mail server. Postini was acquired by Google some time ago and Brightmail by Symantec. Protection is very affordable starting at about $1 per mailbox per month for basic protection on Postini.
The best protection is achieved by a dual approach. That is, using a service that filters mail before it gets to the server and a secondary level at the computer. I have seen mail traffic for a domain of which over 90% of it is unwanted spam and viruses. By using services like Postini and Brightmail, all of this mail is filtered before it even gets to the domain’s mail server. This saves a considerable amount of bandwidth and other system resources.