Understanding the SpamAssassin Filter

SpamAssassin is one of the most widely used spam-filtering software packages It utilizes sets of rules to which it compares the content and makeup of an email and gives a score that indicates how likely the email is spam. The score can be personalized by server admins, with a higher score indicating a higher likelihood of a spam email (for the sake of reference, we will use a simple scale of 1-10 and a threshold of 5). Most emails with a score of less than 5 will be delivered without issue, whereas emails with scores higher than 5 can cause delivery issues that continue to increase along the score. SpamAssassin is quite well documented in regards to the rules they use, but 100% compliance with every rule is nigh impossible. However, as long as the overall SpamAssassin score is under 5, serious delivery problems should be able to be avoided. Some of these rules include excessive capitalization, especially entire lines; maintaining a proper ratio of HTML to text, as some spam attempts to use HTML to circumvent spam filters; even to something as simple as overuse of the dollar sign, e.g., $$$ in the subject.

While these rules are usually applicable only to spam, other rules of SpamAssassin are bound to flag one’s email, for instance: if the email contains a tollfree number, SpamAssassin will flag it; another rule looks for claims that the recipient opted in, which is most likely the case; it even has a rule for how to opt-out of receiving future emails, something that is required by law in many U.S. states. These are a small fraction of the entire rule set that SpamAssassin uses, but one can see that while a score of 1-4 could easily be assigned to any legitimate email, the key is to avoid any major rule infringements that raise the score above 5 and increase delivery troubles.

Because SpamAssassin is an open source software, a sender could acquire SpamAssassin, install it, and set up a test environment with which to test emails. Obviously, because the software is personalizable per server admin, the exact set of rules that the email will encounter is impossible to guess; however, setting up the test environment would allow for at least a general screening for potential issues.

The sheer number of SpamAssassin rules makes avoiding being given a score impossible, as the software does, in some ways, penalize legitimate emails. On the other hand, ensuring that the sender’s score is as low as possible is relatively easy, given how explicitly the rules are documented. For a complete breakdown of the different rules that SpamAssassin performs, check out the official SpamAssassin site. The amount of transparency is great, although you may be overwhelmed by the amount of rules.

Essentially, the open source nature and robust rule set of SpamAssassin directly contribute to its prolific usage among server admins who are responsible for mail distribution. They can put up a customized filter that will automatically take care of a majority of spam. However, there is a high probability that even a normally compliant email will be caught, so it behooves a professional email marketer to reduce any outright spam characteristics to avoid approaching the threshold score set by the server administrator.

Written by
Marcin Chruszcz
Account Manager at ExpertSender
Email and marketing geek. Specializes in email deliverability, strategy and design. Basketball and MTB riding enthusiast.
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