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The SMTPServer class of the smtpd module creates a server socket with the IPv4 socket.AF_INET address family hardcoded, and this prevents it from later binding to an IPv6 local address.
This occurs on line 282 of smtpd.py for the Python 2.7 branch:
http://hg.python.org/cpython/file/5319a4bf72e7/Lib/smtpd.py#l282
And on line 435 of smtpd for the Python 3.2 branch ( Lib/smtpd.py:435 ):
http://hg.python.org/cpython/file/d937b527b76e/Lib/smtpd.py#l435
One IPv4/IPv6 agnostic solution is to look up provided local address with getaddrinfo(), and use one of the result's address family, socket type and address tuple for create_socket() and bind() at those lines:
...
try:
 gai_results = socket.getaddrinfo(localaddr[0], localaddr[1])
 self.create_socket(gai_results[0][0], gai_results[0][1])
 # try to re-use a server port if possible
 self.set_reuse_addr()
 self.bind(gai_results[0][4])
 self.listen(5)
...