How do SpamAssassin HTML_IMAGE_RATIO scores affect email deliverability and how to diagnose outlook 365 spam issues?
Michael Ko
Co-founder & CEO, Suped
Published 1 Jul 2025
Updated 15 Aug 2025
7 min read
Email deliverability can often feel like a puzzle, especially when dealing with nuanced spam filtering scores. One common source of confusion I've encountered revolves around SpamAssassin's HTML_IMAGE_RATIO scores and their perceived impact on inbox placement, particularly with a major recipient like Microsoft Outlook 365.
The rule essentially checks the balance between the visual space taken by images and the textual content in an email. It's a fundamental concept in email content optimization, as overly image-heavy emails are often a red flag for spam filters. For more general information about this, you can look into how the image to text ratio affects deliverability.
However, the direct correlation between these specific SpamAssassin scores and Outlook 365 spam issues isn't always straightforward. It's easy to assume that a high score from one system directly translates to problems with another, but that's rarely the case. We need to distinguish between a third-party spam scoring tool and Outlook's proprietary filtering mechanisms.
SpamAssassin is an open-source spam filtering platform that uses a variety of tests to identify spam. The HTML_IMAGE_RATIO rule (or rules, as there are variations) is one of many designed to catch certain spammer behaviors. It specifically looks at the proportion of HTML image area versus the total HTML body area. A higher image ratio can trigger a higher spam score, contributing to the email being flagged as spam.
When you see scores like HTML_IMAGE_RATIO_04 or HTML_IMAGE_RATIO_06, these represent different thresholds for the image-to-text ratio. For instance, HTML_IMAGE_RATIO_04 might apply if the image area is between 0.2% and 0.4% of the message body, while HTML_IMAGE_RATIO_06 covers a higher range, such as 0.4% to 0.6%. Each rule contributes a certain number of points to the email's overall SpamAssassin score. A higher overall score pushes the email closer to being classified as spam. Learn more about how SpamAssassin rules affect email deliverability.
It's important to remember that SpamAssassin's effectiveness can vary greatly depending on its configuration and the specific rulesets in use. Different installations may apply different weights or even have custom rules. This means a score from one SpamAssassin instance or a third-party tester like Mail-tester.com might not directly correlate with how a major mailbox provider like Outlook will treat your email.
SpamAssassin HTML image ratio rules and their interpretation
Rule Name
Image Area Ratio
Description
HTML_IMAGE_RATIO_02
0.000 - 0.002
Very low image area relative to text, potentially indicating a basic, text-heavy email.
HTML_IMAGE_RATIO_04
0.002 - 0.004
Low image area, but still present. This range is often a slight indicator of spam.
HTML_IMAGE_RATIO_06
0.004 - 0.006
Moderate image area, increasing the likelihood of a higher spam score. Can be legitimate or spammy.
HTML_IMAGE_RATIO_08
0.006 - 0.008
Higher image area, often associated with marketing emails. Higher spam risk depending on other factors.
The Microsoft Outlook 365 filtering ecosystem
A critical point to grasp is that Microsoft Outlook and Office 365 do not use SpamAssassin as their primary or even secondary spam filtering engine. Microsoft employs its own sophisticated, proprietary filtering system, including SmartScreen technology, which continuously evolves. This system leverages a combination of reputation-based filtering, content analysis, machine learning algorithms, and user feedback to determine inbox placement.
Therefore, even if a third-party tool gives your email a good or bad SpamAssassin score, it doesn't directly dictate how Outlook 365 will treat it. The factors Outlook considers for deliverability are far more complex and dynamic, encompassing sender reputation, authentication protocols (SPF, DKIM, DMARC), content quality, engagement metrics, and historical sending patterns.
So, while content balance (like image to text ratio) is certainly a factor Outlook's filters evaluate, it's not based on a static SpamAssassin rule. The challenge with Outlook's proprietary system is that the exact weight of each factor is not publicly disclosed, making diagnosis more challenging. You might experience deliverability issues with Outlook and Hotmail for a myriad of reasons beyond just content ratio.
Instead, when Outlook or Office 365 sends an email to spam, it provides specific diagnostics in the email headers. These are the true indicators of why Microsoft flagged your message.
Diagnosing Outlook 365 spam issues
Diagnosing Outlook 365 spam issues requires a different approach than analyzing SpamAssassin scores. The key is to examine the email headers of messages that land in the junk folder. Microsoftprovides extensive diagnostic information within these headers, which can pinpoint the exact reason for filtering. Relevant fields include X-Forefront-Antispam-Report and X-Microsoft-Antispam.
Within these headers, you'll find crucial scores like BCL (Bulk Complaint Level) and SCL (Spam Confidence Level). The BCL indicates how likely an email is to generate a user complaint, while the SCL assesses the likelihood of a message being spam. A high BCL often means your sender reputation is low, while a high SCL points to content or sending patterns that resemble spam. For instance, an Outlook BCL of 6 is a strong indicator of bulk email issues.
The example below illustrates where you might find these diagnostics:
Relies on analyzing specific rule hits (e.g., HTML_IMAGE_RATIO) from a SpamAssassin scan report. Useful for understanding general content issues if recipient uses SA.
Process: Scan email with a SpamAssassin checker. Identify high-scoring rules.
Relevance: Limited for Outlook 365, as Microsoft doesn't use SA.
Outlook 365 specific diagnosis
Focuses on Microsoft's internal headers, specifically BCL and SCL scores, and other proprietary diagnostics.
Process: Retrieve full email headers from the junked message in Outlook. Analyze X-Forefront-Antispam-Report and X-Microsoft-Antispam for BCL/SCL.
Relevance: Directly applicable and crucial for Outlook 365 deliverability.
Addressing image to text ratio for Outlook
Even though Outlook doesn't rely on SpamAssassin rules, the underlying principle of a balanced image-to-text ratio remains relevant for good deliverability. Emails that are predominantly images with minimal text can still trigger spam filters because this is a common tactic used by spammers to evade text-based content filters. Mailbox providers, including Microsoft, prefer a good balance, as it typically indicates legitimate, informative content.
To improve your chances of reaching the inbox in Outlook 365, aim for a healthy text-to-image ratio. This doesn't mean avoiding images altogether, but rather ensuring your emails have sufficient text content to convey your message. Always use proper HTMLformatting and valid HTML that renders correctly across different email clients.
Testing your email across various email clients and using a comprehensive email deliverability test can provide more accurate insights into how your messages are performing. Focus on holistic deliverability factors rather than getting fixated on one specific score from a non-relevant system.
Content balance best practices
Ratio: Aim for at least 60-40% text to images. More text is generally safer for deliverability. While Microsoft doesn't publish exact thresholds, this is a widely accepted guideline for all mailbox providers.
Alt text: Always use descriptive alt text for all images. This provides context even if images are blocked and can help content filters understand your message.
Text in images: Avoid embedding crucial text within images. If filters can't read it, it's invisible to them, and could be seen as an attempt to hide content.
Views from the trenches
Best practices
Maintain a healthy image-to-text ratio, generally favoring more text, to satisfy a broad range of spam filters, even those not using SpamAssassin.
Prioritize a strong sender reputation through consistent volume, low complaint rates, and proper email authentication (SPF, DKIM, DMARC).
Regularly monitor your email headers for
Microsoft-specific diagnostics, such as SCL and BCL scores, as these are the true indicators of deliverability issues within Outlook 365.
Ensure your email lists are clean and engaged to minimize bounces and spam trap hits, which directly impact sender reputation.
Implement a feedback loop with
Microsoft
Common pitfalls
Over-reliance on general spam checkers like Mail-tester.com for specific mailbox provider diagnosis, especially when dealing with proprietary systems like Outlook 365.
Ignoring email headers from junked messages in
Outlook 365
, as they contain critical insights like SCL and BCL scores that directly explain filtering decisions.
Sending image-only emails or emails with very minimal text, which can universally trigger spam filters regardless of the specific scoring system.
Neglecting sender reputation and authentication, which are often far more influential on deliverability than content-based scores alone.
Failing to segment lists and personalize content, leading to lower engagement and higher complaint rates, negatively impacting deliverability.
Expert tips
Don't chase specific SpamAssassin scores if your primary deliverability issue is with
Outlook 365
. Instead, focus on
Microsoft's
own diagnostic headers.
A low
HTML_IMAGE_RATIO
Marketer view
Marketer from Email Geeks says they noticed two very similar emails received different SpamAssassin scores for HTML_IMAGE_RATIO. The email with HTML_IMAGE_RATIO_04 went to spam, while the one with HTML_IMAGE_RATIO_06 went to the inbox, which seemed counter-intuitive.
2020-03-11 - Email Geeks
Expert view
Expert from Email Geeks says that the HTML_IMAGE_RATIO tests relate to the ratio of text to images in a message, where a higher suffix number indicates a higher image ratio, which should typically mean a higher spam score. They pointed out that if 06 has a higher score, it should be the one going to spam.
2020-03-11 - Email Geeks
Moving forward with deliverability
While SpamAssassin's HTML_IMAGE_RATIO scores can provide general insights into your email content's structure, they are not direct indicators of deliverability issues with Outlook 365. Microsoft uses its own advanced filtering mechanisms that look at a much broader set of factors.
To effectively diagnose and resolve Outlook 365 spam issues, shift your focus to Microsoft's specific diagnostic headers, particularly BCL and SCL scores. These provide the most accurate insights into how Outlook's filters are evaluating your emails.
Ultimately, achieving excellent deliverability requires a holistic strategy that prioritizes strong sender reputation, proper email authentication, engaging and balanced content, and consistent monitoring of your email performance. This comprehensive approach will help you land more messages in the inbox, regardless of which spam filter they encounter.
Microsoft Outlook 365.
Outlook 365. Microsoft uses its own advanced filtering mechanisms that look at a much broader set of factors.
