Definition
Image blocking refers to the default behaviour of email clients that prevents images from loading automatically when a recipient opens a message. Most major clients including Gmail, Outlook, and Yahoo Mail block images by default, requiring the recipient to explicitly choose to display them. This affects approximately 43% of all email opens, where images remain blocked throughout the reading experience.
The practice originated as a privacy and security measure. Early email clients loaded all images automatically, which allowed senders to track opens via embedded tracking pixels and exposed recipients to potential malicious content. Modern image blocking is more sophisticated: Gmail caches images on its own proxy servers and Apple Mail's Mail Privacy Protection loads content in the background, but the fundamental constraint of unreliable image display remains a central consideration for email designers.
Image blocking has profound implications for email analytics. Open rates, which rely on a 1x1 pixel tracking image, become less reliable when images are blocked. Apple's MPP has further complicated this by pre-loading images regardless of actual user engagement. This has pushed the industry toward alternative engagement metrics such as click-to-open rates, conversion tracking, and read time analysis that do not depend on image loading.
Best Practices
Always include descriptive alt text on every image so recipients who block images can understand what would have been displayed. Alt text should describe the image's function, not its appearance, and should be concise enough to read in the image placeholder.
Set background colours on image containers that complement your email design, so the layout remains visually coherent even when images do not load. If your email relies on a white background, set bgcolor="#ffffff" on tables and cells to maintain readability.
Never build an email that relies entirely on images to convey your message. Use live text for headlines, body copy, and calls to action. Reserve images for decorative or supportive content that enhances rather than carries the communication.
Avoid using image-only emails, sometimes called image-flood emails, where the entire message is a single large image. These perform poorly with image blocking, trigger spam filters due to high image-to-text ratios, and are inaccessible to screen readers.
Test your emails with images blocked in each major client to verify that the layout, alt text, and fallback colours create a coherent experience. Tools like Acid, Email on Acid, and Litmus provide previews of how emails appear with images disabled.
Related Glossary Terms
Email Apple Mail
Apple's email client for macOS and iOS, known for the best CSS support among major email clients due to its WebKit rendering engine.
Email Background Image
A background image applied to an email's table cell or container, requiring VML fallbacks for Outlook due to its lack of CSS background-image support.
Email CASL
Canada's Anti-Spam Legislation requirements for commercial electronic messages, including consent types, record-keeping, and penalties up to $10 million per violation.
Email CCPA
California Consumer Privacy Act requirements for email marketing, including consumer rights to access, delete, and opt out of data collection and sale.
Email Conditional
Proprietary conditional comments used to target Microsoft Outlook with specific HTML or VML code for cross-client email rendering.
Email Consent Record
Email consent record-keeping captures proof of opt-in including timestamp, IP address, method, and exact wording shown. GDPR requires controllers to demonstrate consent upon request.
Frequently Asked Questions
Yes, Gmail blocks images by default. Gmail caches images on its own servers before displaying them, which means senders cannot see the recipient's IP address. Recipients can choose to "always display images from sender" on a per-sender basis.
Image blocking makes open rates less reliable because the tracking pixel cannot load if images are blocked. Apple's Mail Privacy Protection has further reduced accuracy by pre-loading images regardless of user behaviour, inflating open counts for Apple Mail users.
No. Image loading is controlled by the recipient's email client settings. You cannot force images to load, but you can encourage a recipient to enable images by providing useful alt text and a positive sender reputation that builds trust.
Gmail, Outlook desktop (2010–2021), Outlook.com, Yahoo Mail, and AOL all block images by default. Apple Mail loads images by default unless the user changes settings. Thunderbird gives users a prompt on first open.
Yes, all image types including JPEG, PNG, GIF, and SVG are subject to image blocking. Tracking pixels (typically 1x1 transparent GIFs) are images and are blocked alongside all other images. No image format bypasses the client's image blocking policy.