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Issue #<3921> may be the same problem.
Sending Gmail by smtplib fails on Python 3.0 and 3.0.1.
It seems to be exist two problems in encode_plain function in smtplib.py:
 * parameter of encode_base64 must be bytes, not str,
 * by default, encode_base64 adds extra newline.
The following is an example of patch.
# original version
def encode_plain(user, password):
 return encode_base64("0円%s0円%s" % (user, password))
# fixed version. Note that "eol=''" is given in Python 2.6's smtplib.
def encode_plain(user, password):
 s = "0円%s0円%s" % (user, password)
 return encode_base64(s.encode('ascii'), eol='')