Antispam Software, article by Eran Shaham, security expert

To prevent e-mail spam, end users and administrators of e-mail systems use various anti-spam techniques. None of the techniques is a complete solution to the spam problem, and each has tradeoffs between incorrectly rejecting legitimate e-mail vs. not rejecting all spam, and associated costs in time and effort.

Anti-spam techniques can be broken into two broad categories: those that require actions by individuals, and those that can be automated.

How to choose the best antispam program and which email defense techniques to use

1. Your antispam application should be configured for automatic functioning. 
   Many Internet service providers and e-mail clients have automated anti-spam systems installed, or can have optional systems added. Since all anti-spam techniques can cause legitimate e-mail to be incorrectly identified as spam, many anti-spam systems are either not enabled by default or are configured to be very conservative about what will be identified as spam. 
2. E-mail address harvesting. Give out your email address with care. Harvesting describes methods that spammers use to obtain e-mail addresses of real people. If the spammers can't get your address, the address is less likely to have spam sent to it. 
   Most people want new people to be able to contact them via e-mail and many people cannot hide their e-mail addresses. While preventing spammers from obtaining email addresses does not solve the spam problem anymore than avoiding the high crime areas of a city solves crime, individuals need to weigh the risks.
3. Address munging. Using a false email address. 
   Posting anonymously, or with a fake name and address, is one way to avoid "address harvesting," but users should ensure that the fake address is not valid. Users who want to receive legitimate email regarding their posts or Web sites can alter their addresses so humans can figure them out, but automated spammers cannot. For instance, joe@example.net might post as joeNOS@PAM.example.net.invalid, or display his email address as an image instead of text. Address mugging, however, can cause legitimate replies to be lost.
4. Use a disposable e-mail address. 
   Email users sometimes need to give an address to a site without complete assurance that the site will not send out spam. One way to mitigate the risk is to provide a disposable email address — a temporary address which forwards email to a real account, which the user can disable or abandon. A number of services provide disposable address forwarding. Addresses can be manually disabled, can expire after a given time interval, or can expire after a certain number of messages have been forwarded.
5. Avoid HTML enabled e-mail. 
   Many modern mail programs incorporate Web browser functionality, such as the display of HTML, URLs, and images. This can easily expose the user to pornographic or otherwise offensive images in spam. In addition, spam written in HTML can contain web bugs which allow spammers to see that the e-mail address is valid and has not been caught in spam filters. JavaScript programs can be used to direct the user's Web browser to an advertised page, or to make the spam message difficult to close or delete.
6. Use mail applications which do not automatically download and display HTML, images or attachments.
7. Avoid responding to spam. 
   Spammers may regard responses to their messages—even responses of "Don't spam me" — as confirmation that an email address is valid. Likewise, many spam messages contain Web links or addresses which the user is directed to follow to be removed from the spammer's mailing list – ignore these.
8. Reporting spam email option. Be aware to whom you report. 
   Tracking down a spammer's ISP and reporting the offense often leads to the spammer's service being terminated. Unfortunately, it can be difficult to track down the spammer — and while there are some online tools to assist, they are not always accurate. Occasionally, spammers employ their own netblocks. In this case, the abuse contact for the netblock can be the spammer itself and can confirm your address.
9. Email application that automatically blocks spam. 
   Mail agents discover patterns and automatically block these mails, based on their characteristics. These can be useful.
10. Email application that lets you decide whether to filter. 
    Statistical filtering, once set up, requires no maintenance. Instead, users mark messages as spam or nonspam and the filtering software learns from these judgments. Thus, a statistical filter does not reflect the software author's or administrator's biases as to content, but it does reflect the user's biases as to content.