Problems with mail encoding

I'm having problems with mail encodings other that UTF-8. Even I've set the default charset for iso-8859-1, it keeps giving me the error shown below. Another question I have is: even if I manage to fix this to work with iso-8859-1, how will mailman be able to process other encoding too, like for example windows-1252, us-ascii or utf-8? Just because not everyone has the e-mail client configured in the same way.
Below is the error. Thanks in advance.
Jan 13 11:01:41 2003 (2277) Uncaught runner exception: unknown encoding Jan 13 11:01:41 2003 (2277) Traceback (most recent call last): File "/var/mailman/Mailman/Queue/Runner.py", line 105, in _oneloop self._onefile(msg, msgdata) File "/var/mailman/Mailman/Queue/Runner.py", line 155, in _onefile keepqueued = self._dispose(mlist, msg, msgdata) File "/var/mailman/Mailman/Queue/IncomingRunner.py", line 129, in _dispose status = self._dopipeline(mlist, msg, msgdata, pipeline) File "/var/mailman/Mailman/Queue/IncomingRunner.py", line 152, in _dopipeline sys.modules[modname].process(mlist, msg, msgdata) File "/var/mailman/Mailman/Handlers/CookHeaders.py", line 74, in process prefix_subject(mlist, msg, msgdata) File "/var/mailman/Mailman/Handlers/CookHeaders.py", line 253, in prefix_subject h.append(s, c) File "/var/mailman/pythonlib/email/Header.py", line 243, in append s = s.encode(outcodec) LookupError: unknown encoding
Jan 13 11:01:41 2003 (2277) SHUNTING: 1042455697.098894+a208992fecd38608c2fb91f0c337e5ed8bbeafba
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