Summary
While email content filters generally focus more on text, links, and sender reputation, images can indirectly trigger spam filters in several ways. Large image sizes, high image-to-text ratios, and missing alt text can negatively impact deliverability. The reputation of the domain hosting the image is also crucial. Sophisticated filters use OCR to scan images for embedded text. Email fingerprinting is a technique where a unique hash is generated from the email's content (including images) for comparison against spam signatures, or to identify similar messages even with slight variations. Fingerprinting is part of a broader analysis that includes sender reputation. Consistent sending patterns and content, reputable image hosting, balanced image/text ratio, and descriptive alt text are key to avoiding spam filters.
Key findings
- Indirect Image Impact: Images themselves aren't the primary trigger but contribute to overall spam assessment.
- Fingerprinting Components: Fingerprinting involves unique hashes based on text, images, and attachments.
- Image Hosting Reputation: The reputation of domains hosting images affects deliverability.
- OCR Scanning: Embedded text within images is scanned by sophisticated filters using OCR.
- Email Stream Analysis: Filters focus on mail streams, links, hostnames, as well as email body text content.
- Fingerprinting Purpose: Fingerprinting helps to detect new spam camapigns and zero-day exploits.
- Consistent Sending: Consistent sending patterns are important.
- Alt Text Needed: Missing or irrelevant alt text is a red flag for spam filters.
Key considerations
- Balance Image and Text: Maintain a healthy image-to-text ratio.
- Use Descriptive Alt Text: Always include descriptive alt text for accessibility and context.
- Reputable Image Hosting: Host images on reputable CDNs or your own domain.
- Monitor Sender Reputation: Proactively monitor your sender reputation.
- Image domain reputation: Check the reputation of any image domains or URLs you use.
- Consider an A/B test: A/B test images versus no images.
What email marketers say
10 marketer opinions
Images in emails can indirectly trigger spam filters through various factors. Large image sizes, high image-to-text ratios, missing or irrelevant alt text, and hosting images on domains with poor reputations can negatively affect deliverability. Modern spam filters use OCR to scan images for embedded text and fingerprinting techniques to identify patterns in email content, sender behavior, and infrastructure. An image's inclusion in previous spam campaigns can also negatively impact its reputation. Balancing image use with sufficient text, using reputable image hosting, and including descriptive alt text are important considerations.
Key opinions
- Image Size Matters: Large images can contribute to deliverability issues, especially with insufficient text.
- Image-to-Text Ratio: High image-to-text ratios can negatively impact deliverability, as filters may see it as an attempt to bypass text-based analysis.
- Alt Text Importance: Missing or irrelevant alt text can be a red flag, hindering accessibility and potentially hiding content.
- Image Hosting Reputation: Using suspicious or blacklisted image hosting sites can trigger spam filters.
- OCR Scanning: Spam filters now commonly scan images for suspicious keywords embedded within them.
- Email Fingerprinting: Fingerprinting identifies and tracks patterns to identify spam campaigns.
- Reputation Transfer: Images included in previous spam complaints inherit negative reputation.
Key considerations
- Balance Image Use: Ensure a balanced image-to-text ratio to provide context and signal legitimacy.
- Use Reputable Hosting: Utilize a reputable CDN or your own domain for image hosting.
- Include Descriptive Alt Text: Always include descriptive alt text for images to improve accessibility and provide context if the image doesn't load.
- Image URL Reputation: Always check the reputation of any image domains or URLs you use.
Marketer view
Email marketer from Email Geeks shares an experience where a client received a fingerprint on an image because the image was included in other messages that drove spam complaints, the data associated with the fingerprint decides if the fingerprint will impact the stream negatively.
7 Jul 2022 - Email Geeks
Marketer view
Email marketer from Reddit r/EmailMarketing shares that using links to suspicious or blacklisted image hosting sites can trigger spam filters, regardless of the image content itself.
17 Mar 2023 - Reddit r/EmailMarketing
What the experts say
9 expert opinions
While spam filters don't heavily analyze image content directly, images play a role in email filtering. Image URLs from domains with poor reputations can negatively affect deliverability. Sophisticated filters might use OCR to analyze text within images. Email fingerprinting, used by services like Cloudmark, condenses emails into short, comparable patterns for content filtering and identifying similar messages or mail streams. This 'lossy' representation allows minor changes without altering the fingerprint. Fingerprinting helps cluster similar emails, treating them as a group in modern mail filtering, and it's a method of vector search.
Key opinions
- Limited Image Analysis: Spam filters don't typically analyze images directly but focus on mail streams, links, and text.
- Image Fingerprinting: Images contribute to the overall message fingerprint used to identify similar content.
- Cloudmark Usage: Cloudmark fingerprints all emails for content filtering.
- Fingerprint as Short Representation: A fingerprint is a condensed pattern making emails easy to compare.
- Lossy Representation: Minor changes to the email body don't alter the fingerprint.
- Clustering Similar Emails: Fingerprinting clusters similar emails together for modern mail filtering.
- Poor Image Reputation: Images hosted on low-reputation domains hurt deliverability.
- OCR scanning of images: Sophisticated filters use OCR to scan for text within images to detect spammy content.
- Fingerprinting Patterns: Email fingerprinting identifies patterns in email content, sender behavior and infrastructure.
Key considerations
- Image Hosting Reputation: Be aware of the reputation of the domains hosting your images.
- Text in Images: Avoid putting text in images which might be scanned by OCR software.
- Consider A/B Testing: A/B test your email to see the impact on similarity score.
Expert view
Expert from Email Geeks shares that fingerprinting is the simplest, most dumbed-down, highest performance implementation of a general multi-dimensional vector search. It is used to cluster similar emails together and treat them as a group, is a universal thing in modern mail filtering.
26 Apr 2022 - Email Geeks
Expert view
Expert from SpamResource shares that while simple image analysis is not typical, sophisticated filters can perform OCR (Optical Character Recognition) on images to detect spammy text embedded within them.
10 Nov 2023 - SpamResource
What the documentation says
5 technical articles
Email fingerprinting, a technique used by various email security systems, involves creating a unique hash or signature of an email's content, including text, images, and attachments. This fingerprint is then compared against known spam signatures or used to identify near-identical messages, even with slight variations. Fingerprinting helps detect spam campaigns and analyze zero-day exploits, and it's often combined with sender reputation analysis to determine if a message is spam. Consistent sending patterns and content are vital for establishing a good sender reputation.
Key findings
- Fuzzy Hashing: SpamAssassin uses fuzzy hashing (similar to fingerprinting) to identify near-identical messages with slight variations.
- Unique Hash Creation: Fingerprinting creates a unique hash of an email's content, including text, images, and attachments.
- Signature Comparison: The generated fingerprint is compared against known spam signatures.
- Holistic Analysis: Email filtering systems analyze various signals, including content fingerprints and sender reputation.
- Hash Value: Email fingerprinting calculates a hash value from different parts of the email, which is used to check if the email is a variant of spam or a known good email.
- Zero-Day Exploit Analysis: Fingerprinting is used to analyze zero-day exploits, including those in images.
Key considerations
- Content Consistency: Maintaining consistent sending patterns and content is crucial for a good sender reputation.
- Sender Reputation: Build and maintain a good sender reputation.
Technical article
Documentation from Microsoft 365 Defender documentation explains that its email filtering system analyzes various signals, including content fingerprints and sender reputation, to determine if a message is spam. Consistent sending patterns and content are key to establishing a good reputation.
30 May 2024 - Microsoft 365 Defender documentation
Technical article
Documentation from SpamAssassin Wiki explains that it uses various techniques, including fuzzy hashing (similar to fingerprinting), to identify near-identical messages. This helps in detecting spam campaigns that use slight variations of the same content.
28 Jan 2024 - SpamAssassin Wiki
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