Summary
Maintaining a balanced image-to-text ratio is crucial for email deliverability, as high ratios can trigger spam filters like SpamAssassin. However, the significance of SpamAssassin scores varies across installations. In Outlook 365, SpamAssassin is not used; instead, analyze email headers for SCL and BCL scores and use the Message Header Analyzer tool. Image optimization, maintaining a good sender reputation through email hygiene, and utilizing feedback loop data from Microsoft's SNDS are also vital for diagnosing and resolving deliverability issues. Pre-testing tools can help identify problems before sending.
Key findings
- Image-to-Text Ratio Matters: High image-to-text ratios negatively affect deliverability, triggering spam filters.
- SpamAssassin Variance: SpamAssassin scoring varies; interpret scores cautiously.
- Outlook 365 Focus: Outlook 365 doesn't use SpamAssassin; header analysis (SCL/BCL) is key.
- Comprehensive Approach: Sender reputation, email hygiene, and SNDS data are crucial components.
- Pre-Testing Benefits: Tools like GlockApps and Email on Acid help identify issues beforehand.
Key considerations
- Balance Content: Ensure a good balance between images and text in your emails.
- Use Appropriate Tools: Utilize pre-testing tools to check spam scores before sending.
- Optimize Images: Compress images to reduce file size and improve loading times.
- Analyze Headers in Outlook: Examine email headers for SCL and BCL scores when troubleshooting Outlook 365.
- Maintain Sender Reputation: Focus on email hygiene and list management to build a positive sender reputation.
- Use Feedback Loops: Incorporate feedback loop data (e.g., SNDS) to monitor spam complaints.
What email marketers say
8 marketer opinions
Maintaining a balanced image-to-text ratio is crucial for email deliverability, as spam filters often flag emails with excessive images and little text. Tools like GlockApps and Email on Acid can help pre-test email deliverability and identify SpamAssassin scores. Optimizing images by reducing file sizes improves deliverability by speeding up loading times. Analyzing email headers in Outlook 365 and using tools like Microsoft's Message Header Analyzer can provide insights into why an email was marked as spam. However, image ratios and spam scores are just part of the bigger picture; maintaining a good sender reputation through email hygiene practices is essential.
Key opinions
- Image-to-Text Ratio: High image-to-text ratios can trigger spam filters.
- Pre-Testing Tools: Tools like GlockApps and Email on Acid can identify SpamAssassin scores.
- Image Optimization: Reducing image file sizes improves deliverability.
- Header Analysis: Analyzing Outlook 365 email headers helps diagnose spam issues.
- Sender Reputation: Good sender reputation is essential for deliverability.
Key considerations
- Balance Image and Text: Ensure a balanced ratio of images to text to avoid spam filters.
- Use Pre-Testing Tools: Utilize tools to identify potential spam triggers before sending emails.
- Optimize Images: Compress images to reduce file sizes and improve loading times.
- Analyze Email Headers: Examine email headers in Outlook 365 to understand spam filtering decisions.
- Maintain Sender Reputation: Practice good email hygiene to maintain a positive sender reputation.
- Image Counts: Pay attention to the total number of images versus text in the email.
Marketer view
Email marketer from Litmus shares that you can use their tool 'Email on Acid' to preview your email and it runs spam tests using popular filters, including SpamAssassin. Helps give you a heads up of potential problems. It will show you the breakdown on the text vs. image ratio
7 Jan 2022 - Litmus
Marketer view
Email marketer from Mailjet Blog explains that a high image-to-text ratio can negatively affect deliverability. Spam filters may flag emails with excessive images and little text as spam because spammers often use images to bypass text-based spam filters. Maintaining a balanced ratio is important.
14 Feb 2025 - Mailjet Blog
What the experts say
4 expert opinions
SpamAssassin's HTML_IMAGE_RATIO scores, while important, should be interpreted cautiously as scoring can vary across different installations. A high image-to-text ratio generally impacts deliverability negatively, so a balanced ratio is recommended. However, for Outlook 365, SpamAssassin scores are irrelevant as it doesn't use this filtering system; instead, examining email headers for SCL and BCL scores is crucial. Utilizing feedback loop data via Microsoft's SNDS system also provides valuable insights into spam and deliverability problems.
Key opinions
- SpamAssassin Score Variance: SpamAssassin scoring varies across installations.
- Image-to-Text Ratio Impact: High image-to-text ratio negatively impacts deliverability.
- Outlook 365 & SpamAssassin: Outlook 365 does not use SpamAssassin.
- Outlook 365 Header Analysis: Examine email headers for SCL/BCL scores in Outlook 365.
- Feedback Loop Data: Feedback loop data via SNDS provides deliverability insights.
Key considerations
- Contextualize SpamAssassin: Interpret SpamAssassin scores cautiously due to score variations.
- Balance Image and Text: Maintain a balanced image-to-text ratio to improve deliverability.
- Focus on Outlook Headers: Analyze email headers, specifically SCL and BCL scores, for Outlook 365 issues.
- Utilize SNDS: Incorporate feedback loop data from Microsoft's SNDS to monitor deliverability.
Expert view
Expert from Spamresource.com explains that a high image-to-text ratio in emails can significantly impact deliverability, as spam filters often flag such emails as potential spam. They recommend maintaining a balanced ratio to avoid triggering these filters.
28 Jul 2022 - Spamresource.com
Expert view
Expert from Word to the Wise, shares that feedback loop (FBL) data gives a lot of information around spam, complaints and deliverability issues. Microsoft provides a SNDS system which contains the data.
23 Feb 2024 - Word to the Wise
What the documentation says
4 technical articles
SpamAssassin's HTML_IMAGE_RATIO tests evaluate the ratio of image area to text in an email, assigning higher scores when the image area is large, potentially indicating spam. In Outlook 365, the Spam Confidence Level (SCL) indicates the likelihood an email is spam, with higher scores leading to filtering. The Message Header Analyzer tool helps examine email headers to identify spam filtering rules, while Bulk Complaint Level (BCL) thresholds determine how likely a message is bulk email, influencing its handling.
Key findings
- HTML_IMAGE_RATIO: SpamAssassin scores increase with larger image area relative to text.
- SCL Score: Higher SCL scores in Outlook indicate a higher likelihood of spam.
- Message Header Analyzer: This tool helps identify specific spam filtering rules triggered.
- BCL Thresholds: BCL indicates the likelihood a message is bulk email, influencing its handling.
Key considerations
- Balance Image to Text: Ensure a reasonable image-to-text ratio to avoid SpamAssassin flags.
- Monitor SCL Scores: Regularly check SCL scores in Outlook to diagnose deliverability issues.
- Utilize Header Analyzer: Use the Message Header Analyzer to pinpoint specific spam filtering triggers.
- Configure BCL Thresholds: Adjust BCL thresholds to properly handle bulk emails without being flagged as spam.
Technical article
Documentation from Microsoft details Bulk Complaint Level (BCL) thresholds in Microsoft 365. BCL indicates how likely a message is to be a bulk email (newsletter, promotion, etc.). Lower BCLs are considered less likely to be spam. Admins can configure actions based on BCL thresholds, influencing how bulk emails are handled.
15 Feb 2025 - Microsoft Learn
Technical article
Documentation from Microsoft Purview explains to use the Message Header Analyzer tool to examine email headers in detail. This tool can help identify specific spam filtering rules that triggered the spam classification. Access the analyzer in the Security & Compliance Center, submit email headers, and the tool will identify the different header values.
11 May 2024 - Microsoft Purview
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