Failed to send notification email: unencrypted connection

[opelx]

Describe the bug

Using the notification type email, on unencrypted (private) network sending mail fails, even with NOTLS. The error is:

Failed to send notification email: unencrypted connection

To Reproduce
Steps to reproduce the behavior:

use docker-compose.yml
--------------------8<-----------------------
version: '3'
services:
watchtower:
image:
containrrr/watchtower
restart: always
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
command:
--cleanup
environment:
- TZ='Europe/Berlin'
- WATCHTOWER_NOTIFICATIONS=email
- WATCHTOWER_NOTIFICATION_EMAIL_FROM=watchtower@example.com
- WATCHTOWER_NOTIFICATION_EMAIL_TO=admin@example.com
- WATCHTOWER_NOTIFICATION_EMAIL_SERVER=mail.example.com
- WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT=587
- WATCHTOWER_NOTIFICATION_EMAIL_SERVER_TLS_SKIP_VERIFY
- WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER=watchtower@example.com
- WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD=watchtower
- WATCHTOWER_NOTIFICATION_EMAIL_DELAY=3
-------------------->8----------------------
$ sudo docker-compose up -d

Expected behavior
Sending an email :)

Environment

• Platform
  clear linux

• Architecture
  i86/x64

• Docker version
  $ docker --version
  Docker version 19.03.6, build 369ce74a3ce86a392e39e45d3960ce970fdfac97
  $ docker-compose --version
  docker-compose version 1.24.1, build unknown

Logs from running watchtower with the --debug option

time="2020-04-03T15:25:28Z" level=info msg="Starting Watchtower and scheduling first run: 2020-04-03 15:30:28 +0000 UTC m=+300.461220288"
Failed to send notification email: unencrypted connection

Additional context
Add any other context about the problem here.

The setup is part of a private "Home Lab" without SSL connections yet, which isn't required at this time. Testing mail transfer works, I'm using mailu.io for this.

[github-actions]

Hi there! 👋🏼 As you're new to this repo, we'd like to suggest that you read our code of conduct as well as our contribution guidelines. Thanks a bunch for opening your first issue! 🙏

[piksel]

This is unfortunatly not supported by the go smtp library:
https://godoc.org/net/smtp#PlainAuth

[opelx]

This is unfortunatly not supported by the go smtp library:
https://godoc.org/net/smtp#PlainAuth

Ok, so far I understand (or even not), What is the purpose of WATCHTOWER_NOTIFICATION_EMAIL_SERVER_TLS_SKIP_VERIFY in this context?

[piksel]

Skipping the certificate verification, meaning you can use a self-signed cert

[opelx]

Ok, thanks.