Email Size Limit

Definition

Email size limits are restrictions imposed by mailbox providers and email clients on the total size of messages they will accept or fully display. Two distinct types of size limits exist: transport limits (the maximum total message size including attachments that an MTA will accept) and rendering limits (the maximum HTML source size before clipping or truncation occurs in the email client). Gmail enforces a 25MB total message limit including attachments and a 102KB HTML source clipping limit. Outlook limits total messages to 25MB and applies a 34KB preview pane clipping in its desktop client. Yahoo also enforces a 25MB total limit.

The 102KB Gmail clipping limit is the most consequential size threshold for email marketers because it applies to the HTML source code size, not the rendered size. Images hosted externally contribute minimal HTML source size, but inline CSS, repeated HTML structures, and Base64-encoded images can quickly push the HTML source over 102KB. When Gmail clips a message, recipients see a truncated email with a "View entire message" link, and all content below the clip point receives dramatically reduced visibility. Outlook's 34KB preview pane clip is similarly impactful for recipients using the Outlook desktop preview pane rather than opening the full message.

Optimisation strategies focus on reducing HTML source size without sacrificing design quality. Image compression using JPEG quality 60–80 reduces file size with minimal visual difference. CSS inlining increases HTML size compared to embedded stylesheets, so the size impact of inlining must be weighed against the rendering benefit. Minimal HTML techniques such as replacing nested tables with simpler structures, removing redundant CSS declarations, and using shorthand CSS properties can reduce source size by 20–40% compared to unoptimised templates.

Best Practices

Keep total email HTML source under 95KB to provide a safety margin below Gmail's 102KB clipping threshold. Measure your actual HTML file size after CSS inlining, not the pre-inlined template size.

Compress JPEG images to quality 60–80 for email use. PNG images should use maximum compression. Never use Base64-encoded images in HTML; they inflate source size by 33% and defeat Gmail's clipping optimisation.

Remove unused CSS declarations after inlining. Many email templates include dozens of CSS rules for elements that never appear in the specific campaign, adding unnecessary kilobytes.

Limit the use of nested tables in HTML layout. Each nesting level adds HTML markup overhead. Modern email designs using simpler table structures can reduce total HTML size by 15–30%.

Test your email size before each send using a tool that reports both total message size and HTML source size. Monitor both Gmail clipping risk and total attachment limits.

Avoid combining large attachments with text-heavy HTML. If attachments bring the total message near 25MB, consider hosting the attachment on your website and linking to it instead.