AFAIK, the email address should support non-ASCII character (from https://stackoverflow.com/questions/760150/can-an-email-address-contain-international-non-english-characters and SMTPUTF8 
 option from https://docs.python.org/3/library/smtplib.html#smtplib.SMTP.sendmail)

>>> import smtplib
>>> s = smtplib.SMTP(host='smtp-mail.outlook.com', port=587)
>>> s.verify('你好@outlook.com')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/windson/learn/cpython/Lib/smtplib.py", line 577, in verify
    self.putcmd("vrfy", _addr_only(address))
  File "/Users/windson/learn/cpython/Lib/smtplib.py", line 367, in putcmd
    self.send(str)
  File "/Users/windson/learn/cpython/Lib/smtplib.py", line 352, in send
    s = s.encode(self.command_encoding)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 5-6: ordinal not in range(128)

I found this issue when I updating https://github.com/python/cpython/pull/8938/files

Btw, from the docs https://docs.python.org/3/library/smtplib.html#smtplib.SMTP.sendmail

> msg may be a string containing characters in the ASCII range, or a byte string. A string is encoded to bytes using the ascii codec, and lone \r and \n characters are converted to \r\n characters. A byte string is not modified.
 
So we can't send non-ASCII msg using send_mail(), is this expected behavior?

Barry: would you mind to have a look at this issue? It seems like smtplib encodes Unicode commands to ASCII, whereas verify() argument (SMTP "VRFY" command) is an hostname and hostname can be non-ASCII ("punycode" encoding, RFC 3492).

Any update?