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.