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Affected python version: 3.5 and above (did test them all except 3.9)
Steps to reproduce:
 from mail.message import EmailMessage
 from mail.policy import SMTP
 msg = EmailMessage(policy=SMTP)
 msg['To'] = 'Joe <joe@examplé.com>' # notice the é in the domain
 print(msg.as_string())
It prints
 To: "Joe <joe@=?utf-8?q?exampl=C3=A9?=.com>"
But it should be
 To: "Joe <joe@xn--exampl-gva.com>"
While b64/qp can be used to encode most non-ascii headers, the domain part of an email address is an exception. According to IDNA2008 (rfc5890 , rfc5891), non-ascii domain should be encoded using the punycode algorithm and the ACE prefix.

Duplicate of https://bugs.python.org/issue39757 

Woops wrong copie/paste, here is the correct link: https://bugs.python.org/issue11783 

This is not actually a duplicate of 11783. Rereading (parts of) that issue, we decided we currently have no good way to do automatic conversion between unicode and internationalized domains, so the user of the library has to do it themselves. This means that the bug *here* is that the new email API is *wrongly* encoding the non-ascii in the domain by using an encoded word. I'm surprised at that; I thought I'd guarded against it.
What should be happening here is that an error should be raised when that header is set (or possibly when it is accessed/serialized, but when set would be better I think) saying that there is non-ascii in the domain part.

Reproduced on 3.11.