Definition
Email content blocking refers to the various ways that email clients, webmail providers, and security gateways modify, truncate, or block elements of an email message to protect recipients or manage rendering. Common blocking types include Gmail clipping (emails over 102KB are truncated with a "View entire message" link), image blocking by default in clients such as Outlook and Gmail (images are not downloaded until the recipient clicks a "Display images" prompt), link wrapping and rewriting by corporate security gateways, and attachment blocking for executable files and certain archive formats.
Gmail's 102KB clipping threshold is one of the most impactful content blocks for email marketers. When the HTML source of an email exceeds this size, Gmail truncates the message and inserts a link to view the full message. This means recipients never see content below the clip point, including calls to action and footer information. Image blocking, while less common than it was a decade ago, still affects clients that do not download images by default. Link rewriting, where corporate security systems replace original links with proxy URLs for threat scanning, can break tracking and disrupt the user experience.
Script blocking is universal in email. No major email client executes JavaScript for security reasons, so any interactive functionality requiring scripts must be implemented through alternative techniques such as AMP for Email or server-side interactivity. Form elements within email suffer from inconsistent support; Gmail and Apple Mail support basic forms in some configurations, but Outlook and Yahoo do not. Understanding which content types are blocked by each provider is essential for designing emails that render and function correctly across the entire subscriber base.
Best Practices
Keep email HTML source under 102KB to avoid Gmail clipping. Use minimal HTML techniques, compress inline CSS, and remove unnecessary whitespace to reduce file size.
Always specify width and height attributes on images to maintain layout when images are blocked. Include descriptive alt text so recipients understand the content even without images.
Avoid executable attachments including .exe, .bat, .msi, and .scr files. Use a secure download link hosted on your website instead of attaching these file types.
Do not rely on JavaScript or embedded forms for critical functionality. Use server-side landing pages for interactive experiences and link to them from the email.
Test emails with a link scanner or security gateway simulator to verify that link wrapping does not break your tracking URLs or redirect paths.
Use progressive enhancement: design emails that function correctly with all content blocks active, then enhance the experience when blocks are not present.
Related Glossary Terms
Email Authentication
Email authentication (SPF, DKIM, DMARC) is a set of technical protocols that verify an email is genuinely from the domain it claims to be sent from. It prevents spoofing, phishing, and improves deliverability.
Email Password Reset
Security-critical transactional emails that enable verified account access recovery, balancing strict authentication requirements with user-friendly design and deliverability.
Email Size Limit
Email size limits including Gmail 25MB total and 102KB HTML clipping, Outlook limits, and optimisation strategies for staying under thresholds.
Frequently Asked Questions
Gmail inserts a truncation point at approximately 102KB of HTML source. All content after the truncation point is hidden behind a "[Message clipped] View entire message" link. Recipients must click the link to see the full email, which reduces visibility of content below the clip point.
Image blocking does not prevent delivery but affects engagement tracking and user experience. Open rate tracking is impacted because opens are measured by image downloads. Apple Mail's Mail Privacy Protection further complicates this by preloading images regardless of user preference.
Corporate email security gateways replace original links with proxy URLs to scan destination pages for malware and phishing threats before the user clicks. This link rewriting can break click tracking if the proxy does not properly forward analytics parameters.
Generally no. Most email providers block ZIP file attachments because they can contain executable malware. Gmail, Outlook, and Yahoo all block or reject messages with ZIP attachments unless they are password-protected (which some providers also block).
Basic HTML forms (text inputs, checkboxes, submit buttons) work in Apple Mail, Gmail, and some mobile clients. However, Outlook desktop uses Word's rendering engine which does not support form elements. Vast majority of senders use linked landing pages rather than in-email forms for reliable cross-client support.