How to Avoid Being Filtered by SpamAssassin

07 March 2022

The amount of emails that are spam has declined significantly in the last 10 years, but still represents nearly half of all email sent.

You may not have been aware that it’s that high, and one reason those emails never make it to your inbox is due to email filtering software. Tools like SpamAssassin analyze incoming emails and assign scores, and they are filtered out . Email clients then put these these messages into a spam folder, saving the effort of looking at them.

This is immensely beneficial for the email users, as it helps eliminate everything from unwanted advertising to malicious phishing attempts.

However, for email marketers, this presents a challenge. A lot of email marketing is legitimate and opted in, but can still be flagged as spam, and never seen.

In this article, we explain what SpamAssassin is, and how to prevent being filtered by it.

spam in the total e-mail traffic worldwide

Source: Statista – Monthly share of spam in the total e-mail traffic worldwide from January 2014 to December 2023

What is SpamAssassin?

SpamAssassin is open-source software, first launched in 2001, and developed by the Apache Foundation, which makes Apache HTTP. Apache is used to serve nearly 1/3 of websites on the internet.

Using an algorithm and analytics to evaluate email content, SpamAssassin determines gives a score based on multiple criteria such as message content and sender reputation.

As free and open-source software, SpamAssasin is widely adopted. It is used by everyone from system administrators to large email platforms, effectively blocking large amounts of global spam messages from ever being seen.

These are some of the criteria that SpamAssasin uses:

  • Sender’s IP address
  • Subject line content
  • Email headers
  • Use of authentication
  • Body content
  • Use of markup languages
  • Character set used
  • Message is encoding
  • Presence on blocklists
  • DKIM and SPF record configuration
  • Suspicious links and attachments
  • Spam related terms
  • Use of disallowed scripts

What SpamAssassin tests look for

SpamAssassin scans emails and runs tests to look for attributes and patterns associated with spam. There are over 700 tests that SpamAssassin uses to detect spam, and a variety of techniques including Bayesian filtering, blacklists such as Spamhaus, and DNS.

How SpamAssassin scores email

Each individual message gets its own spam score. The lower the score, the better. Anything above 5 is considered spam.

Preliminary scoring is given for each of the hundreds of attributes that SpamAssassin checks. Individual attribute scores are added to give an overall SpamAssassin score.

  • Negative numbers are actually positive. This indicates the email is unlikely to be spam
  • 0 is neutral
  • Positive numbers suggest possible spam

The maximum possible score is 10, however, system administrators and mail providers occasionally modify the threshold, setting it either lower or higher than 5. So it is best to stay well below 5.

How to Avoid SpamAssassin

Generally speaking, email that is is not spam should not result in a bad SpamAssassin score. But if your SpamAssassin score is higher than you’d prefer, here are a few suggestions to reduce the score and your likelihood of being filtered out.

Establish a Good Reputation

To be recognized as a legitimate sender, displaying accurate sender information—such as your “from” and “reply-to” addresses—is essential. Ensure your domain has a verifiable IP address that recipients can check to confirm your identity. Sender reputation is established with each email sent and weighs on establishing a history for your domain and IP address. End user engagement with your emails also affects your reputation, while being marked as spam or ignored can have detrimental impact.

Avoid Coding Errors in HTML Emails

HTML errors, such as invalid tags and default titles, can lead to emails being classified as spam. Clean HTML code is essential. Additionally, for HTML emails, include a plain text version for recipients who prefer that format. Avoid invisible text, as it can also trigger spam filters.

Authenticate Emails

Email authentication helps email servers distinguish legitimate senders from scammers. SpamAssassin assesses emails based on SPF (Sender Policy Framework) and DKIM – DomainKeys Identified Mail.

The absence of either can negatively impact your score. Thus, email authentication crucial for maintaining a low SpamAssassin score. DMARC enhances security, it doesn’t directly influence SpamAssasin score, but its use is still recommended for added protection.

Multiple Images in Email Negatively Affect Score

Using one or two images in your email is generally acceptable, but overuse can be a problem. SpamAssassin flags emails that are primarily composed of images. While this may not drastically alter the score, it does factor in. Minimizing images to less than 40% of an email message by byte size is necessary. Also, it’s necessary to always include alt text in images. Alt text is key to usability and accessibility, and affects reputation.

Language Matters – A Lot

Spam messages tend to use common language, which will trigger filters. Be sure to avoid using spammy language anywhere in the message or title. Some recognizable examples:

  • “This email is not a spam”
  • Winner, Win
  • Save
  • Weight loss
  • Profit
  • Best price
  • Cheap
  • No questions asked
  • Free
  • Money
  • Rich
  • Income
  • Debt

These are common terms in marketing copy, so if you need to use them, try to find alternate words and language, and use good grammar and writing form.

Be Careful With Links in Emails

Marketing email is all about links. You want conversion, but be careful what you link to. The domain reputation of sites you link to will affect your own. If you are linking out to sites that are blacklisted as malicious, or scammy, you will be seen as coordinating to drive malicious traffic.

And it isn’t always obvious. Companies that appear legitimate may be on blacklists for poor security or IT practices.

To Summarize

Monitoring your own sender reputation is very useful for understanding and predicting whether your emails will be filtered. Additionally, this can provide useful feedback on whether specific campaigns may have triggered spam filters, to diagnose and avoid in the future.

But enerally speaking, just maintaining good practices and good faith will avoid being flagged as spam.

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