Keep messages small

How to do it

Gmail clips HTML bodies larger than about 102 KB and hides the rest, including any unsubscribe link at the bottom. Many servers refuse messages above 10 to 25 MB, and base64 encoding adds about a third to attachments. Host images and large files, and link to them. Images do not count towards the clipping limit, but inline styles, tracking codes and long links do. Remove unused CSS and minify the HTML when you build it.

Example

const htmlBytes = Buffer.byteLength(renderedHtml, 'utf8');
if (htmlBytes > 100 * 1024) throw new Error(`HTML part is ${htmlBytes} bytes, keep it under 100 KB`);

html_bytes = len(rendered_html.encode("utf-8"))
if html_bytes > 100 * 1024:
    raise ValueError(f"HTML part is {html_bytes} bytes, keep it under 100 KB")

How to check

A script sends each template and fails when the encoded HTML part passes 100 KB or the whole message passes 5 MB. A test message in Gmail shows no "Message clipped" notice.