[MM3-users] Re: Mail control not given from Mailman 3 to postfix

To follow up, I received the traceback and the dump of the shunted message off-list. The traceback is

Jul 07 21:36:47 2020 (26815) Uncaught runner exception: sequence item 0: expected str instance, Header found
Jul 07 21:36:47 2020 (26815) Traceback (most recent call last):
  File "/var/lib/mailman/mailman/src/mailman/core/runner.py", line 173, in _one_iteration
    self._process_one_file(msg, msgdata)
  File "/var/lib/mailman/mailman/src/mailman/core/runner.py", line 266, in _process_one_file
    keepqueued = self._dispose(mlist, msg, msgdata)
  File "/var/lib/mailman/mailman/src/mailman/runners/pipeline.py", line 37, in _dispose
    process(mlist, msg, msgdata, pipeline)
  File "/var/lib/mailman/mailman/src/mailman/core/pipelines.py", line 50, in process
    handler.process(mlist, msg, msgdata)
  File "/var/lib/mailman/mailman/src/mailman/handlers/avoid_duplicates.py", line 59, in process
    addrs = getaddresses(msg.get_all(header, []))
  File "/usr/lib64/python3.6/email/utils.py", line 112, in getaddresses
    all = COMMASPACE.join(fieldvalues)
TypeError: sequence item 0: expected str instance, Header found
Jul 07 21:36:47 2020 (26815) SHUNTING: 1594138007.5613732+1a5932f41fb4cfcd18c9b0b0ac6f999c84408577

This was apparently caused by an invalid RFC 2047 encoded To: header which encoded the entire header including the address as one base64, unknown-8bit character set encoded word. However, despite trying with several different Python versions, I was unable to recreate the TypeError, so the reason for this exception remains unknown.