rfc822 module: deleting headers

[Jason R. Mastaler]

This question concerns how to remove headers from an incoming mail
message using the rfc822 module.  The docs say:

    Message instances also support the mapping writable interface
    m[name] = value and del m[name].

So take the following program invoked from a .qmail file (qmail's
equivalent of .forward):
    
    #!/usr/bin/env python
    import cStringIO, rfc822, sys

    stdin = cStringIO.StringIO(sys.stdin.read())
    headers = rfc822.Message(stdin)
    body = stdin.read()

    print headers.headers
    del headers['Message-ID']
    print headers.headers

    sys.exit(0)

As you can see, I attempt to remove the 'Message-ID' header before
exiting 0, which causes qmail to skip to the next line in my .qmail
file, which writes the message to my mailbox.

However, when the message reaches my mailbox, it still contains
'Message-ID'.  The two print statements above show that indeed 
Message-ID was removed from the headers list, but this isn't showing
up in the message.

So how is this falling out of sync?  I assume because of the exit(0)?
Any way to synchronize the rfc822.Message instance with the actual
message headers before the program exits?

Thanks.