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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.