Added support for LOGIN AUTH for the SMTP sender

[santanoce]

This should fix #1020, even though it seems that Microsoft 365 Outlook is no longer allowing base authentication for personal accounts, so I wasn't able to actually test it with a Microsoft account. I did however test it using a simple SMTP server supporting LOGIN AUTH and it works.

I basically followed the suggestion in #930, more specifically using the code from here, which in turn is an adaptation of code in the net/smtp package.

[binwiederhier]

I am quite interested in merging this, but I don't know how to test it.

[santanoce]

I am quite interested in merging this, but I don't know how to test it.

@binwiederhier, you may use Docker Mailserver. For example:

services:
  mailserver:
    image: mailserver/docker-mailserver:latest
    hostname: mail.example.com
    container_name: mailserver
    ports:
      - "587:587"
    volumes:
      - ./dovecot.cf:/tmp/docker-mailserver/dovecot.cf
    environment:
      - ENABLE_SPAMASSASSIN=0
      - ENABLE_CLAMAV=0
      - ENABLE_FAIL2BAN=0
      - [email protected]
    cap_add:
      - NET_ADMIN
    restart: always

and inside dovecot.cf you can force only the LOGIN AUTH mechanism:

auth_mechanisms = login
disable_plaintext_auth = no

The application needs an account to start with, that you can create with the following command:

docker exec -ti mailserver setup email add [email protected] mypassword

Now you can check the available authentication mechanism via nc, for example:

nc localhost 587
EHLO localhost

At this point ntfy can be configured with:

smtp-sender-addr: "localhost:587"
smtp-sender-user: "[email protected]"
smtp-sender-pass: "mypassword"

Now you should be able to check that the LOGIN authentication method is actually working. Indeed, if you try to send a message without my commits you will receive an Invalid authentication mechanism error from ntfy logs, while everything will work fine if you try with the commits from this PR.