Inbox Insights: A Deep Dive into SpamAssassin’s Email Filtering

In the vast world of emails, approximately 45% of those sent in 2022 were categorised as spam. Thanks to advanced email filtering software like SpamAssassin, users are spared from sifting through these unwanted messages. For email marketers and agencies like O’Brien Media, ensuring that emails land in the recipient’s inbox and not the spam folder is crucial for successful email marketing campaigns.

Deciphering SpamAssassin

SpamAssassin, a creation of the Apache Foundation, is an open-source software that employs intricate algorithms and analytical tools to scrutinise emails. It determines the likelihood of an email being spam based on various factors such as content, sender reputation, and more. It’s a powerful tool used by system administrators and email clients to filter out and classify emails, preventing spam from reaching people’s inboxes.

The Scoring Mechanism

Every email processed by SpamAssassin receives a score, indicating the probability of it being spam. A score above 5 is generally considered spam, but this threshold can be adjusted by system administrators and mailbox providers. Therefore, it’s essential to aim for a score well below 5 to ensure email deliverability.

Strategies to Work within SpamAssassin Rules

For legitimate senders, a few best practices can help in reducing the SpamAssassin score and avoiding being misclassified as spam:

1. Establish Your Legitimacy

Provide accurate sender information and maintain a verifiable IP address. Building a positive sender reputation through consistent recipient interaction is also vital.

2. Perfect Your HTML

Avoid HTML errors and ensure your email has a text section along with the HTML version. Refrain from including invisible text.

3. Implement Email Authentication

Set up SPF and DKIM to lower your SpamAssassin score. Although DMARC doesn’t directly impact the score, it’s recommended for enhanced security.

4. Be Mindful of Image Usage

Limit the number of images to below 40% of the total message size and always include alt text for them.

5. Maintain a Professional Tone

Avoid using spammy words and maintain a normal tone, grammar, and structure in your emails.

6. Be Cautious with Links

Linking to domains with poor sender reputations can adversely affect your own reputation, so be selective with the sites you link to in your emails.


Navigating through SpamAssassin’s intricate filtering system can be challenging, but with the right knowledge and practices, legitimate senders can ensure their emails reach the intended inboxes. At O’Brien Media, we specialise in crafting effective email marketing strategies, ensuring your messages stand out and reach their destination. Whether you need assistance with website design, development, or email marketing, our team is here to guide you through the digital landscape.