(Python 2.3.4, Linux Debian unstable)

The email module's as_string() method generates
messages that do not include a trailing CRLF on the
last line.  This causes problems when Python-created
messages are piped to procmail and delivered to an mbox.

The attached test script illustrates this behavior. 
You must have a working procmail configured to deliver
mail to an mbox (the default configuration will work).
 If you then read the resulting mailbox with /bin/mail,
it appears as if there is only one message in the
mailbox, instead of two.  The second is concatenated on
to the end of the first.  The mbox does not contain a
blank line between the first message and the second. 
Pop servers require this blank line delimiter between
messages.

You could argue that this is procmail's responsibility,
but as far as I can tell from reading RFC 2822, each
line in an email message must terminate in CRLF, and
Python's email module is violating that spec.