Do images in emails trigger spam filters and how does email fingerprinting work?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 Jun 2025
Updated 17 Aug 2025
7 min read
One of the most common questions I hear about email deliverability is whether images in emails can trigger spam filters. It is a valid concern, especially for marketers and businesses that rely heavily on visual content to engage their audience. The truth is, it is not as simple as a yes or no answer.
Spam filters are sophisticated, constantly evolving systems that analyze numerous factors to determine an email's legitimacy. While images themselves rarely act as direct spam triggers, the way they are used, their ratio to text, and how the email's overall content is perceived can certainly influence filtering decisions. Alongside this, a less understood but critical aspect is email fingerprinting, which can classify similar emails as spam, even with minor variations.
Understanding these dynamics is crucial for ensuring your emails consistently land in the inbox.
Images and spam filters: common misconceptions
It is a common misconception that simply including images in an email will automatically send it to the spam folder. In reality, most modern email clients and spam filters are designed to handle images without an inherent bias against them. They recognize that images are a standard part of contemporary email design, essential for branding, engagement, and conveying information visually.
However, problems arise when images are used improperly or excessively. A key factor is the image-to-text ratio. Emails that are composed almost entirely of images, with very little text, can raise red flags for spam filters. This is because spammers often use image-only emails to bypass text-based content filters, hiding malicious links or trigger words within the image itself. Email filters cannot read the content within images, making such emails suspicious.
Another consideration is how images are embedded. While both embedded images and linked images are generally acceptable, embedding large images directly can significantly increase the email's file size. This can slow down loading times and make the email seem suspicious to some filters. It is generally recommended to optimize image file sizes and use external hosting when possible.
Furthermore, ensuring your images have descriptive alt text is not just a matter of accessibility, but also deliverability. Alt text provides context for spam filters, which cannot "see" the image itself. If images are disabled by default in a recipient's inbox, alt text ensures that the message's intent is still conveyed. This transparency can positively influence how your email is perceived by filtering algorithms.
Best practices for images in emails
Balance text and images: Aim for a good ratio, ensuring sufficient text content alongside your visuals.
Optimize image sizes: Compress images to reduce file size and improve loading speed.
Use alt text: Always include descriptive alt attributes for all images.
Avoid image-only emails: Emails consisting solely of a single large image are highly suspicious to filters.
Understanding email fingerprinting
Email fingerprinting is a technique used by spam filters to uniquely identify and classify email content. It involves creating a concise, digital signature (or fingerprint) for each incoming email message. This fingerprint is a short representation of the entire email's content, allowing for fast and efficient comparison of messages.
The primary goal of fingerprinting is to identify mail streams. A mail stream refers to a group of emails that are considered similar, often originating from the same sender or containing identical (or near-identical) content. For example, if you send a marketing campaign to 100 subscribers, all those emails, despite minor personalization, might generate the same fingerprint because they are fundamentally the same message.
Systems like Cloudmark, which evolved from earlier projects like Vipul's Razor, use these fingerprints as a content filter. They do not just fingerprint bad mail, but rather all mail passing through their network. The reputation associated with a specific fingerprint, built from historical data like user complaints and engagement, determines whether subsequent emails with that same fingerprint are delivered or marked as spam (or junk).
Example of a content fingerprint (MD5 hash)
e.g., d41d8cd98f00b204e9800998ecf8427e
How images contribute to fingerprints
While the content of images themselves might not be directly scanned in detail by most spam filters, the URLs that link to those images are absolutely a part of the email's fingerprint. If you host your images externally, the domains and paths used in those image URLs contribute to the overall digital signature of your email. If these URLs are associated with past spam complaints or known malicious activity, it can negatively impact your email's deliverability.
Some advanced spam filters might employ Optical Character Recognition (OCR) technology to scan text within images. This is a more resource-intensive process, but as caching and processing capabilities improve, it may become more common. However, even if OCR is used, the main concern would be if that extracted text contains spam trigger words or suspicious phrases, not simply the presence of text in an image.
It is important to remember that a fingerprint itself is a neutral identifier. It is the historical data associated with that fingerprint – the good and the bad – that determines its impact on deliverability. If a specific fingerprint (or mail stream) has a history of high complaint rates, low engagement, or association with known spam campaigns, subsequent emails sharing that fingerprint will be more likely to be blocklisted (or blacklisted) or sent to the junk folder, regardless of minor content changes.
Content-based filtering
Examines the body of the email for suspicious words, phrases, and formatting. Over-reliance on images with minimal text, or using too many attachments, can trigger these filters. Obfuscated text within images might be a flag if OCR is employed. The actual words and keyword use remain critical.
Reputation-based filtering
Focuses on the sender's reputation (IP address, domain, authentication records like SPF, DKIM, and DMARC). Email fingerprints fall under this category as they help consolidate the reputation of a "mail stream." If a specific fingerprint has a poor history of user complaints or spam reports, future emails with that same fingerprint will be blocked or filtered, regardless of their individual content quality.
Strategies for image and fingerprint management
Maintaining a healthy balance between visual appeal and deliverability is key. While engaging visuals are important for modern email campaigns, they should always complement your message, not replace it entirely. Prioritize clear, concise text, and use images strategically to enhance, rather than overwhelm, the content.
Regularly monitoring your email deliverability and sender reputation is also vital. This includes keeping an eye on your bounce rates, complaint rates, and whether your domain or IP address appears on any email blocklists (or blacklists). A strong sender reputation helps ensure that even emails with images are viewed favorably by filters.
Proper email authentication, such as SPF, DKIM, and DMARC, also plays a significant role. These protocols verify your sender identity, building trust with email providers like Google and Yahoo, and ensuring that your mail streams are recognized as legitimate. Even if images are present, strong authentication helps filters accurately assess the sender's trustworthiness.
Factor
Impact on deliverability
Notes
Image-to-text ratio
High ratio (image-only emails) increases spam score.
Ensure enough plain text content.
Image file size
Very large files can slow loading and be seen as suspicious.
Optimize and compress images for web.
Alt text
Missing alt text can reduce accessibility and context for filters.
Always add descriptive alt text.
Image URLs in fingerprint
URLs linked to images are part of the email's fingerprint.
Ensure image hosting domains have good reputation.
Views from the trenches
Best practices
Always include sufficient plain text content to complement your images.
Use alt text for all images to provide context for both users and filters.
Optimize image file sizes to avoid unnecessarily large email payloads.
Monitor your sender reputation and mail streams regularly for any anomalies.
Common pitfalls
Sending image-only emails with minimal or no accompanying text content.
Ignoring the reputation of domains hosting your images, especially if external.
Assuming that minor changes to an email will avoid triggering a previous bad fingerprint.
Not understanding how email authentication (SPF, DKIM, DMARC) impacts fingerprint trust.
Expert tips
Remember that a fingerprint identifies a 'mail stream', and it's the reputation of that stream that drives filtering decisions.
While OCR on images is possible, it's generally expensive and not the primary method for most filters to detect spam from images alone.
If your image URLs are linked to bad data (e.g., spam complaints from other campaigns), they will negatively affect your stream's reputation.
A/B testing email designs with and without images can help identify specific deliverability impacts.
Marketer view
Marketer from Email Geeks says that images themselves should not cause spam filter issues with big or well-known providers, and the main concern should be accessibility if text within an image does not truly need to be an image.
2024-07-10 - Email Geeks
Expert view
Expert from Email Geeks notes that spam filters do not inspect image content extensively; they focus more on mail streams, links, hostnames, and text within the body. However, the URLs that link to images are definitely used to fingerprint mail.
2024-07-10 - Email Geeks
Key takeaways for reliable deliverability
Images in emails do not inherently trigger spam filters, but their improper use can contribute to deliverability issues. Factors like a high image-to-text ratio, large file sizes, and lack of alt text can make an email appear suspicious. More critically, how images are linked (via their URLs) contributes to an email's unique fingerprint, which spam filters use to identify mail streams and assess their historical reputation.
For optimal deliverability, focus on creating well-balanced emails with sufficient text, optimized images, and strong authentication. By understanding how spam filters and fingerprinting systems evaluate email content holistically, you can craft campaigns that are both visually appealing and highly likely to reach your audience's inbox.
Google and Yahoo, and ensuring that your mail streams are recognized as legitimate. Even if images are present, strong authentication helps filters accurately assess the sender's trustworthiness.
