Milter to fix badly formatted MIME messages

fixmime.py

#!/usr/bin/env python3
 
import email
from email.mime.multipart import MIMEMultipart
import sys
from subprocess import Popen, PIPE
 
def reformat(msg):
    """Reformat the MIME parts of the message to a sane one."""
    newmsg = MIMEMultipart()
    for name, value in msg.items():
        if name not in ['Content-Type', 'MIME-Version',
                        'Content-Length', 'Lines']:
            newmsg.add_header(name, value)
    textpart = None
    htmlpart = None
    attachments = []
    for part in msg.walk():
        if part.get_filename():
            attachments.append(part)
        elif part.get_content_type() == 'text/plain':
            textpart = part
        elif part.get_content_type() == 'text/html':
            htmlpart = part
    content = MIMEMultipart('alternative')
    if textpart:
        content.attach(textpart)
    if htmlpart:
        content.attach(htmlpart)
    newmsg.attach(content)
    for a in attachments:
        newmsg.attach(a)
    return newmsg
 
 
def main():
    with sys.stdin:
        oldmsg = email.message_from_file(sys.stdin)
    assert oldmsg.is_multipart()
    newmsg = reformat(oldmsg)
    print(newmsg.as_string())
 
def content_filter():
    with sys.stdin:
        oldmsg = email.message_from_file(sys.stdin)
    newmsg = reformat(oldmsg)
    p = Popen(["/usr/sbin/sendmail", "-G", "-i"] + sys.argv[1:],
              stdin=PIPE)
    p.communicate(newmsg.as_string().encode('utf-8'))
 
if __name__ == '__main__':
    content_filter()

main.cf

smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/sender_access
local_header_rewrite_clients = permit_mynetworks

master.cf

fixmime  unix  -       n       n       -       10      pipe
  flags=Rq user=nobody null_sender=
  argv=/usr/local/bin/fixmime.py -f ${sender} -- ${recipient}

sender_access

from.address.of.misbehaving.appliance@domain.org        FILTER fixmime:dummy

This was written some time ago to fix e-mail reports generated by Vmware vSphere Data Protection Appliance. It's authors ignored MIME standards, which resulted in:

• Date header missing (this can be fixed in MTA)
• Text and HTML parts are being sent in wrong order, so proper MUA always renders plaintext version only
• Optional CSV attachment is attached as third alternative body, rendering it instead HTML or text content

As far as I know this appliance is now discontinued. But there may be others that would get use of such milter.