Gmail clips an HTML message that exceeds roughly 102 KB on the desktop web client. The clip is destructive: the visible body ends at the truncation point, everything after it is replaced with a “[Message clipped] View entire message” link, and the recipient has to click that link and load the full source in a new tab before any content, CTA, or tracking pixel below the cut can render or fire. Almost nobody clicks it. If your CTA or open pixel sits below the cut, they simply do not exist for that opening. The 102 KB budget is the byte length of the message body Gmail is asked to render – not the file size of the delivered .eml – so it includes every character of markup, every inline style repetition, every base64-encoded image, every HTML comment, every tracking parameter on every href, and every <style> block. Templates that look fine in a browser preview clip because the browser rendered a decoded document while Gmail measures the raw source.
The WordPress Email Authority