Since smtplib.quoteaddr() uses email.utils.parseaddr(), and the patch for issue 11783 fixes email.utils.parseaddr(), that patch will hopefully solve this issue as well (though a test case wouldn't hurt for sure).

What I had not realised is that hostnames are also used elsewhere, in particular in the ehlo() and helo() but also in connect(). Do you consider that a separate issue or part of this issue?

Are there other places where you think a fix is needed?

I may be able to create a patch, though bear with me: I never checked out the source for Python or the standard library (other than installing point releases through my package manager).