postfilter.py

#!/usr/bin/env python3
# vim: fileencoding=utf-8 tw=100 expandtab ts=4 sw=4 :

import sys
import subprocess
from email.parser import Parser
from email.utils import parseaddr, formataddr


# ---
# Custom filter
# ---


# This header is added to all mails to avoid processing the same email twice. Your postfix
# configuration should at all times prevent this from happening, as messages going through filters
# are removed from the queue and not sent. However, if by accident it happened, then this prevents
# postfix from going in a crazy infinite loop.
BRANDING_HEADER = 'X-Fix-From'


def apply_filter(frm, to, content):
    # TODO: write your filter in here
    return frm, to, content


# ---
# Boilerplate code
# ---


EX_TEMPFAIL = 75
EX_UNAVAILABLE = 69


def parse_args():
    try:
        cli_from = sys.argv[1].lower()
        cli_to = [x.lower() for x in sys.argv[2:]]
    except IndexError:
        sys.exit(EX_UNAVAILABLE)
    else:
        return cli_from, cli_to


def get_content():
    content = ''.join(sys.stdin.readlines())
    return Parser().parsestr(content)


def re_inject(frm, to, content):
    if BRANDING_HEADER in content:
        return True

    content[BRANDING_HEADER] = 'yes'

    p = subprocess.Popen(
        ['/usr/sbin/sendmail', '-G', '-i', '-f', frm] + to,
        stdin=subprocess.PIPE,
        stdout=subprocess.DEVNULL,
        stderr=subprocess.DEVNULL,
    )

    p.communicate(content.as_bytes())
    ret = p.wait()

    if ret == 0:
        return True

    return False


def main():
    frm, to = parse_args()
    content = get_content()

    if not re_inject(*apply_filter(frm, to, content)):
        sys.exit(EX_TEMPFAIL)


if __name__ == '__main__':
    main()