What does moz-do-not-send do?

Question

I see the moz-do-not-send="true" attribute in the HTML of emails all the time. Whenever I send links in emails with Thunderbird, it adds it to the <a> tag. What does the moz-do-not-send="true" attribute do?

When I looked it up, I couldn't find a clear answer, but instead suggestions to use it when writing email HTML with no explanation of what it does.

Answer 1

Since at least 2004, Thunderbird has been designed by default to include the full image file contents of images inserted in composed emails. If moz-do-not-send is absent or false, then upon sending the email, Thunderbird downloads the image and attaches it to the email.

The moz-do-not-send attribute is a signal used internally by Thunderbird to tell it not to include the full image file contents, but instead to simply link to the external image. It's a proprietary attribute used internally by Mozilla-derived software; it has never been part of any HTML specification.

This is best documented at MozillaZine here. You can see details of the use of this attribute in the Thunderbird source repository here.

Answer 2

Cant add a comment, due to the 50 reputation restriction.. Just need to alert to the fact that the checked "solution" is a bit off-topic. All the answers here refer to <img> tags which is not what the OP asked about. The question refers to anchor tags. ie <a moz-do-not-send="true" href="https://blah.blah">blah.blah</a> So what does moz-do-not-send do for an anchor in an email?