Note that I've used undocumented Message class in the example instead of EmailMessage, which could also be used. The reason for it is that urllib.requests internally uses Message, so for consistency it may be better to use it. Message has also been very stable (15+ years) and so very unlikely to be renamed or removed.

I've added a code example because it may be confusing to just direct users to use Message when the parameter obviously requires just the headers.

https://bugs.python.org/issue45572