Attached mail, parsed with email.message_from_binary_file results in:

Traceback (most recent call last):
  File "./mail_filter.py", line 616, in <module>
    ret = main.run()
  File "./mail_filter.py", line 605, in run
    self.process(fp)
  File "./mail_filter.py", line 589, in process
    self.save_message(msg, self._fname + '.out')
  File "./mail_filter.py", line 103, in save_message
    ofd.write(msg.as_bytes())
  File "/usr/lib64/python3.4/email/message.py", line 179, in as_bytes
    g.flatten(self, unixfrom=unixfrom)
  File "/usr/lib64/python3.4/email/generator.py", line 115, in flatten
    self._write(msg)
  File "/usr/lib64/python3.4/email/generator.py", line 195, in _write
    self._write_headers(msg)
  File "/usr/lib64/python3.4/email/generator.py", line 422, in _write_headers
    self._fp.write(self.policy.fold_binary(h, v))
  File "/usr/lib64/python3.4/email/policy.py", line 190, in fold_binary
    folded = self._fold(name, value, refold_binary=self.cte_type=='7bit')
  File "/usr/lib64/python3.4/email/policy.py", line 204, in _fold
    return self.header_factory(name, ''.join(lines)).fold(policy=self)
  File "/usr/lib64/python3.4/email/headerregistry.py", line 255, in fold
    return header.fold(policy=policy)
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 300, in fold
    self._fold(folded)
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 1228, in _fold
    rest._fold(folded)
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 338, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 149, in append_if_fits
    token._fold(self)
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 324, in _fold
    for part in self.parts:
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 254, in parts
    if token.startswith_fws():
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 267, in startswith_fws
    return self[0].startswith_fws()
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 267, in startswith_fws
    return self[0].startswith_fws()
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 267, in startswith_fws
    return self[0].startswith_fws()
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 267, in startswith_fws
    return self[0].startswith_fws()
  File "/usr/lib64/python3.4/email/_header_value_parser.py", line 267, in startswith_fws
    return self[0].startswith_fws()
IndexError: list index out of range

when flattened with BytesGenerator.