Now that the wrap_fixed=False is set, it looks like no messages are actually changed by the rewrapping.
I think this is because the decode() method splits lines using b'\r\n', i.e. CRLF However it looks like all the mbox files use just LF as their line endings. So decode will always return the body as a single chunk to convertToWrapped() which then does nothing unless wrap_fixed=True.
