IAB Releases Guidelines for Email Deliverability - Dance of the Seven Filters
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The IAB identified no less than seven different filtering methods used by ISPs and corporate system administrators to make sure their clients don't have to deal with unwanted email. If you want your email to make it to its destination, you need to know how these filters work. You may not be able to do much about some of them, but others you can definitely avoid triggering.
Blacklists, public and private, account for one of the seven filtering methods. You can find out easily enough if you're on a public blacklist such as the ones maintained by Spamhaus or Spam Cop; they are, after all, public. These are lists of the IP addresses of suspected spammers. If you find yourself on a public blacklist, the web sites usually explain what you can do to get yourself removed from their list. Private blacklists are maintained by ISPs and some corporations for internal use. They're often based on user complaints, but may also be based on other criteria.
Fingerprinting and machine learning-based content filters account for two more filtering methods; both are content-based. The latter technique looks at words and phrases in the body or header of the email, and is commonly used for client-side filters. Fingerprinting, used by Brightmail, matches known spam content with new email and filters based on how closely they match.
Spam traps are email addresses that are used specifically to attract spam. Maybe the address was active at one time, but hasn't been used in years; or maybe it was specifically created to attract spam. Either way, a lot of anti-spam organizations monitor spam traps.
Other filtering techniques include setting a volume cap on connections for sending email; "challenge response" to prove you are a real person (used by SpamArrest among others); and not accepting email from those whose servers are not configured correctly.
Next: How to Avoid Looking Like Spam >>
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