With regard to msg349895, is there any chance this fix could be considered for backport?

I imagine you could view it as a new feature, but it seems to be the only official fix we have for the fact that Python 3 generates invalid SMTP messages. And that's not a minor problem because many popular MTAs (GMail, Outlook, etc.) will rewrite non-RFC-conformant Message IDs, causing the original ID to be lost and missing in subsequent replies. This breaks an important mechanism to support email threads.

To this day, several Linux distributions still ship 3.6 or 3.7, even in their latest LTS, and users and vendors are stuck with supporting those for a while.

Thanks!