Mailman3's LMTP is failing on non-utf8 incoming email
As explained on IRC, Python 3.4's SMTPD server is decoding incoming bytes as utf-8 and will fail on incompatible encodings, closing the channel. As a result, I get this message on Mailman3's standard output:
error: uncaptured python exception, closing channel <mailman.runners.lmtp.Channel connected 127.0.0.1:57680 at 0x7f09823de9b0> (<class 'UnicodeDecodeError'>:'utf-8' codec can't decode byte 0xa0 in position 1725: invalid start byte [/usr/lib64/python3.4/asyncore.py|read|83] [/usr/lib64/python3.4/asyncore.py|handle_read_event|442] [/usr/lib64/python3.4/asynchat.py|handle_read|167] [/usr/lib64/python3.4/smtpd.py|collect_incoming_data|290])
And then Mailman does not accept any new email from Postfix and has to be restarted. No mail is lost, but it will happen again when Postfix retries the message.
This bug seems to have been fixed in Python 3.5, here's a way to port the fix to 3.4: http://bugs.python.org/issue19662#msg210433