Python program that pretends to be sendmail, sends to Slack webhook instead. Move to /usr/lib/sendmail and make executable (`chmod +x`)

sendmail_slack.py

#!/usr/bin/env python3
import sys
import json
import requests

slack_url = "https://hooks.slack.com/services/<webhook URL>"


def get_subject(lines):
    try:
        # Find subject
        for line in lines:
            if line.startswith('Subject:'):
                subject = line.split(':', 1)[1]
                return subject.strip()
    except Exception as e:
        print('get_subject exception', e)
        return ''


def get_body(lines):
    try:
        # Find body
        for i, line in enumerate(lines):
            # first blank line
            if line == '\n':
                body_lines = lines[i + 1:]
                # list to string
                return ''.join(body_lines)
    except Exception as e:
        print('get_body exception', e)
        return ''

# Read from stdin until there is no more
lines = []
for line in sys.stdin:
    lines.append(line)

subject = get_subject(lines)
body = get_body(lines)

text = f'*{subject}*\n{body}'
slack_data = {'text': text}

# Fire and forget
requests.post(
    slack_url, data=json.dumps(slack_data),
    headers={'Content-Type': 'application/json'}
)