Drop-in replacement for sendmail -t which relays to gmail smtp, should be easy to adapt to any smtp

mail_relay.py

#!/usr/bin/env python3

import email
import smtplib
import sys

smtp_address = "smtp.gmail.com"
smtp_port = 587
smtp_user_name = "[email protected]"
smtp_password = "YOUR_APP_PASSWORD"

with smtplib.SMTP(smtp_address, smtp_port) as smtp:
    smtp.starttls()
    smtp.login(smtp_user_name, smtp_password)
    msg = email.message_from_string(sys.stdin.read())
    smtp.send_message(msg, from_addr=smtp_user_name)