smtplib has limited support for non-ASCII domain names in the From to To mail address. It only works for punycode-encoded domain names, submitted as unicode string (e.g. server.rcpt(u"user@xn--e1afmkfd.ru").

The following two calls fail:

server.rcpt(u"user@пример.ru"):
  File smtplib.py, line 332, in send
    s = s.encode("ascii")
UnicodeEncodeError: 'ascii' codec can't encode character '\u03c0' in position 19: ordinal not in range(128)
http://hg.python.org/cpython/file/3.3/Lib/smtplib.py#l332

server.rcpt(b"user@xn--e1afmkfd.ru"):
  File email/_parseaddr.py, line 236, in gotonext
    if self.field[self.pos] in self.LWS + '\n\r':
TypeError: 'in <string>' requires string as left operand, not int
http://hg.python.org/cpython/file/3.3/Lib/email/_parseaddr.py#l236

There are three ways to solve this (from trivial to complex):
* Make it clear in the documentation what type of input is expected.
* Accept punycode-encoded domain names in email addresses, either in string or binary format.
* Accept Unicode-encoded domain names, and do the punycode encoding in the smtplib if required.

See also 

References:
https://tools.ietf.org/html/rfc5891: Internationalized Domain Names in Applications (IDNA): Protocol