10.12 Sierra Mail not rendering some emails correctly
This seems to be related to an image rendering bug in the new version of Apple Mail (Sierra).

When an &lt;img&gt; element in an HTML mail doesn't have both a "width" and a "height" attribute, the image is not displayed the first time the email is opened. If both attributes are defined, the image is properly displayed. If either one or both are missing, the image is not displayed.

The same &lt;img&gt; element is properly rendered in previous versions of Apple Mail.