In Python 2.6 PLAIN authentication works, in Python 3.1 not. Lib/test/test_imaplib.py does not test IMAP4.authenticate() or IMAP4.login_cram_md5() functions, only IMAP4.login(). I would still like to go back to imaplib._Authenticator.encode() function. The function is below. # inp = authobject(response) def encode(self, inp): oup = '' while inp: if len(inp) > 48: t = inp[:48] inp = inp[48:] else: t = inp inp = '' e = binascii.b2a_base64(t) if e: oup = oup + e[:-1] return oup binascii.b2a_base64() takes bytes, so inp must therefore be bytes, and returns bytes (Python 3). Then str + bytes (out + e[:-1]) fails. The fix would then be changing oup = oup + e[:-1] to oup = oup + e[:-1].decode()