Comment 3 for bug 265962

As far as I recognised mailman copies Message.py from the
python libraries so I also reported it for python (id 854102).

As a workaround replace the line in question:

return unicode(newvalue[2], newvalue[0] or 'us-ascii')

with

try:
  return unicode(newvalue[2], newvalue[0] or 'us-ascii')
except LookupError:
  return unicode(newvalue[2], 'us-ascii', 'replace')