What is SpamAssassin and how does it score emails?

SpamAssassin assigns scores to emails based on a set of predefined rules. The higher the score, the more likely the email is considered spam. While it can influence delivery for smaller mail servers, major mailbox providers like Gmail and Yahoo use more sophisticated, holistic filtering systems that consider factors beyond simple rule hits.

Do SpamAssassin rules directly affect deliverability to major email providers?

For major mailbox providers, SpamAssassin's direct impact is limited. They rely on advanced algorithms that prioritize sender reputation, authentication protocols (SPF, DKIM, DMARC), and recipient engagement. SpamAssassin can still be relevant for self-hosted mail servers.

What are some common SpamAssassin rules and what do they indicate?

Examples include rules for suspicious TLDs (like KAM_INFOUSMEBIZ ), subject lines ending in numbers ( KAM_NUMSUBJECT ), and imbalances in image-to-text ratios. While these contribute to a SpamAssassin score, their individual impact on modern deliverability varies.

What are the most important factors for email deliverability beyond SpamAssassin?

Focus on improving your sender reputation, ensuring proper email authentication (SPF, DKIM, DMARC), maintaining a clean and engaged email list, and crafting high-quality, relevant content. These factors have a much greater impact on inbox placement.

What is considered a good SpamAssassin score?

A good SpamAssassin score (below 5.0) can be beneficial, especially if your recipients use mail servers that rely on it. However, a perfect score doesn't guarantee inbox delivery with major ISPs, nor does a slightly higher score doom your emails if other deliverability factors are strong.

How do spamassassin rules affect email deliverability? - Technical - Email deliverability - Knowledge base

Email deliverability is a complex topic, and many factors determine whether your messages land in the inbox or the spam folder. One tool often discussed in this context is SpamAssassin, an open-source email filter that assigns a score to incoming emails based on a set of rules. The higher the score, the more likely an email is considered spam. This raises a crucial question for senders: How much do SpamAssassin rules truly affect email deliverability in today's sophisticated email ecosystem?

While SpamAssassin can provide valuable insights for smaller mail servers and self-hosted domains, its direct impact on deliverability to major mailbox providers like Gmail and Yahoo is often overstated. These large providers utilize their own advanced proprietary filtering systems that go far beyond simplistic rule-based scoring. Understanding the nuances of SpamAssassin's role can help you focus your efforts on what truly matters for inbox placement.

Understanding SpamAssassin's scoring system

SpamAssassin works by running numerous tests on an email's headers and body, each associated with a positive or negative score. These scores are then tallied to produce a final numerical value. If this score exceeds a predefined threshold, typically 5.0, the email is flagged as spam or rejected. For example, a rule might assign a score of 0.8 KAM_INFOUSMEBIZ if your domain uses certain TLDs that have been historically associated with spam.

These rules cover a wide array of potential spam indicators, from suspicious phrases and formatting to technical issues like missing authentication headers. It's a comprehensive system designed to catch a broad spectrum of unwanted mail. Different configurations of SpamAssassin may also include custom rules, like the KAM_ rules mentioned in some discussions, which are additional aggressive checks implemented by third-party products.

The scores are additive, meaning that an email can accumulate points from multiple rule hits. Even a small score from one rule, like 0.5 KAM_NUMSUBJECT for subject lines ending in numbers, contributes to the overall score. While this specific rule might not be a major concern on its own, a combination of such low-scoring hits can push an email over the spam threshold.

SpamAssassin versus modern mailbox provider filters

While SpamAssassin provides a useful framework for identifying spam characteristics, its direct influence on email deliverability, especially with major internet service providers (ISPs) and mailbox providers, has significantly diminished over time. Most large providers today rely on a more holistic approach to spam filtering. Their systems analyze thousands of signals, including sender reputation, engagement metrics, and historical sending patterns, rather than just relying on fixed rules.

This means that even if your email triggers a SpamAssassin rule, it doesn't automatically mean it will land in the spam folder at Gmail or Outlook. Their internal algorithms are constantly evolving and are much more dynamic. However, SpamAssassin remains relevant for smaller mail servers, self-hosted domains (e.g., those using Plesk), and some niche email systems that might still employ it as their primary spam filter.

While a low SpamAssassin score is generally desirable, obsessing over every minor rule hit might be misdirected effort for those targeting major mailbox providers. It's more beneficial to view SpamAssassin test results as a diagnostic tool rather than a definitive judgment on your deliverability. They can point to potential content or formatting issues that, while not directly blocking your email at a major ISP, could still contribute to a perception of spamminess by recipients or less sophisticated filters.

SpamAssassin's rule-based filtering

Static rules: Relies on a predefined set of rules, each with a specific score, to identify spam characteristics. Rules are updated but not in real-time based on live email streams.
Content-focused: Primarily evaluates email content, headers, and basic technical configurations like the From address alignment.
Local impact: More relevant for individual or smaller mail servers that have SpamAssassin installed.

Modern mailbox provider filtering

Dynamic algorithms: Uses machine learning and artificial intelligence to analyze vast amounts of real-time data, including sender reputation, user engagement, and network traffic patterns.
Holistic evaluation: Considers sender authentication (SPF, DKIM, DMARC), spam complaints, engagement rates, and content quality simultaneously.
Broad impact: Determines inbox placement for the majority of email traffic globally (Gmail, Outlook, Yahoo).

Analyzing common SpamAssassin rule hits

Let's delve into some common SpamAssassin rules and what they signify. The KAM_INFOUSMEBIZ rule, for instance, flags emails if they use certain generic Top-Level Domains (gTLDs) like .info, .us, .me, or .biz. While a .me domain is perfectly legitimate, these specific TLDs have historically been more prone to abuse by spammers. If you're using such a domain, it's worth checking its reputation with trusted sources like Spamhaus. If it's not listed as abused, the SpamAssassin score might be less concerning.

Another common rule is KAM_NUMSUBJECT, which triggers if your subject line ends in numbers, excluding current years. While this might seem arbitrary, it's often a pattern observed in malware and phishing emails (e.g., "Your Invoice #12345"). This specific rule typically carries a low score, suggesting it's not a major issue on its own. However, it's always a good practice to review your subject line formats and ensure they don't inadvertently mimic known spam patterns, regardless of whether you are trying to avoid spam trigger words.

Other SpamAssassin rules can focus on the ratio of HTML to image content, broken links, or even the inclusion of long hexadecimal URIs. These content-based issues can impact perceived quality across various spam filters, not just SpamAssassin. Addressing them contributes to overall email health.

Best practices for content and format

Subject lines: Craft clear, concise subject lines that avoid suspicious patterns, excessive capitalization, or unusual characters.
Image-to-text ratio: Maintain a healthy balance of text and images. Emails with too many images and little text can trigger spam filters.
Link quality: Ensure all links are valid, reputable, and not unnecessarily long or obfuscated. Broken or suspicious links can severely impact deliverability.
HTML and plain text: Always include a well-formatted plain text version of your email. Mismatched HTML and text parts can raise a red flag.

Beyond SpamAssassin: key deliverability factors

While SpamAssassin scores can offer some preliminary checks, focusing solely on them risks missing the bigger picture of modern email deliverability. Today, the most critical factors influencing whether your emails reach the inbox are sender reputation, robust email authentication, and recipient engagement.

Sender reputation, encompassing both your IP and domain, is paramount. Mailbox providers track your sending history, including spam complaint rates, bounce rates, and whether your IPs or domains appear on any blocklists (or blacklists). A consistently poor reputation will lead to emails being sent directly to spam or rejected outright, regardless of your SpamAssassin score.

Email authentication protocols like SPF, DKIM, and DMARC are fundamental. They verify that your emails are legitimate and prevent spoofing. Properly configured DMARC records, for instance, signal to receiving servers that your domain is serious about security and email deliverability. Without these, even well-intentioned emails risk rejection. For example, a basic SpamAssassin report might look like this:

Example SpamAssassin Report Snippetemail

X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-03-09) on mail.example.com
X-Spam-Flag: YES
X-Spam-Score: 5.8
X-Spam-Tests: KAM_INFOUSMEBIZ,KAM_NUMSUBJECT,HTML_IMAGE_RATIO_08
X-Spam-Status: Yes, score=5.8 required=5.0 tests=KAM_INFOUSMEBIZ,KAM_NUMSUBJECT,
	HTML_IMAGE_RATIO_08 autolearn=no autolearn_force=no version=3.4.6

Finally, recipient engagement plays a significant role. High open rates, clicks, and replies, coupled with low unsubscribe and spam complaint rates, tell mailbox providers that your emails are valued. Conversely, a lack of engagement, or frequent spam reports, will negatively impact your sender reputation and lead to poor inbox placement. Understanding how factors like spam complaints affect deliverability is crucial.

Views from the trenches

Best practices

Maintain strong sender reputation by consistently sending valuable content and monitoring engagement.

Implement robust email authentication (SPF, DKIM, DMARC) to verify your sender identity and build trust with ISPs.

Regularly clean your email lists to remove inactive users and prevent hitting spam traps.

Design emails with a balanced text-to-image ratio and ensure all links are valid and clear.

Pay close attention to user feedback, including unsubscribe rates and spam complaints, to refine your sending practices.

Common pitfalls

Over-optimizing email content solely based on SpamAssassin scores, neglecting broader deliverability factors.

Failing to implement DMARC, leading to potential spoofing and authentication failures.

Sending emails to unengaged recipients, which can trigger spam complaints and damage sender reputation.

Ignoring obscure SpamAssassin rule hits if major mailbox providers are delivering your emails successfully.

Using generic or suspicious top-level domains without validating their current reputation.

Expert tips

While most major providers do not rely on SpamAssassin for primary filtering, it can still indicate underlying issues.

Custom SpamAssassin rules, such as those with the `KAM_` prefix, are not universal.

Focus on content quality and user engagement above minor SpamAssassin rule adjustments.

Subject line patterns that include numbers can sometimes mimic malicious emails, so consider alternatives.

Always check the reputation of your domain's TLD on a blocklist (or blacklist) service.

Expert view

Expert from Email Geeks says that mainstream mailbox providers do not block based on simplistic SpamAssassin rules; their filtering is much more holistic.

2021-01-22 - Email Geeks

Expert view

Expert from Email Geeks says that the KAM_ rules are not standard SpamAssassin rules but are additional aggressive rules from specific third-party products.

2021-01-22 - Email Geeks

Navigating your email deliverability journey

While SpamAssassin can be a helpful diagnostic tool, especially for smaller mail servers, its influence on overall email deliverability to major providers is often minimal. These giants in the email space prioritize a sender's reputation, robust authentication, and how recipients interact with their emails.

My recommendation is to use SpamAssassin reports as an indicator of potential content or formatting issues, rather than a definitive blocklist (or blacklist) score. Address any significant rule hits, particularly those related to content quality or technical misconfigurations, but don't let minor scores distract you from the bigger picture.

True email deliverability success in 2025 hinges on maintaining a pristine sender reputation, consistently sending authenticated emails, and focusing on engaging your audience with valuable content. These are the cornerstones of reaching the inbox reliably.