Header folding is very different (non-existent as far as I've found so far) in Python3.  Here's a short example:

#!/usr/bin/python
# -*- coding: ISO-8859-1

from email.header import Header

hdrin = 'Received: from mailout00.controlledmail.com (mailout00.controlledmail.com [72.81.252.19]) by mailwash7.pair.com (Postfix) with ESMTP id 16BB5BAD5 for <bcc@kitterman.com>; Sun, 13 Mar 2011 23:46:05 -0400 (EDT)'

print(Header(hdrin))

With python2.6 the output is:


Received: from mailout00.controlledmail.com (mailout00.controlledmail.com
 [72.81.252.19]) by mailwash7.pair.com (Postfix) with ESMTP id 16BB5BAD5 for
 <bcc@kitterman.com>; Sun, 13 Mar 2011 23:46:05 -0400 (EDT)

With python3.1 or 3.2 the output is one line:

Received: from mailout00.controlledmail.com (mailout00.controlledmail.com [72.81.252.19]) by mailwash7.pair.com (Postfix) with ESMTP id 16BB5BAD5 for <bcc@kitterman.com>; Sun, 13 Mar 2011 23:46:05 -0400 (EDT)

This makes it very difficult to write header processing code that works for both Python2 and Python3 even if one can fold headers at all in Python3.