Cannot send e-mail from watchtower but can from shoutrrr

[kwijbrans]

When trying to configure the sending of e-mail in watchtower I get the error:

time="2021-12-18T13:26:50Z" level=info msg="Watchtower 1.3.0\nUsing notifications: smtp\nChecking all containers (except explicitly disabled with label)\nScheduling first run: 2021-12-18 13:27:50 +0000 UTC\nNote that the first check will be performed in 59 seconds"
Failed to send notification via shoutrrr (url=smtp://watchtower%40example.com:[email protected]:587/?auth=Plain&fromaddress=watchtower%40example.com&fromname=Watchtower&subject=Watchtower+updates+on+05bdc4d8eea0&toaddresses=admin%40example.com):  failed to send using smtp: timed out

When I try to send the same via the shoutrrr container, it seems to add a starttls option in the URL it shows. With starttls=No it does not work. If I include startls=Yes in the URL, it does work!

$ docker run --network=host 794834b14730 send -u="smtp://watchtower%40example.com:[email protected]:587/?auth=Plain&fromaddress=watchtower%40example.com&fromname=Watchtower&subject=Watchtower+updates+on+e80ba93d5794&toaddresses=admin%40example.com" -m="test" -v
URLs:
  smtp://watchtower%40example.com:[email protected]:587/?auth=Plain&fromaddress=watchtower%40example.com&fromname=Watchtower&subject=Watchtower+updates+on+e80ba93d5794&toaddresses=admin%40example.com
Message: test
SHOUTRRR 2021/12/18 13:39:44 Mail successfully sent to "[email protected]"!
Notification sent

The configuration in docker-compose for watchtower looks like:

  watchtower:
    image: containrrr/watchtower
    container_name: watchtower
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    command:
      - --interval=60
      - --monitor-only
#      - --debug
      - --notifications=email
      - --notifications-level=info
      - [email protected]
      - [email protected]
      - --notification-email-server=mail.example.com
      - --notification-email-server-port=587
      - [email protected]
      - --notification-email-server-password=abc123

Using -- debug does not help, it does not show additional information in watchtower.

To make it even curiouser: it is probably a networking problem. The above docker compose file does not work. The below docker run command does work. Watchtower sends an e-mail when started up via docker run.

docker run -v  /var/run/docker.sock:/var/run/docker.sock --name watchtower containrrr/watchtower --interval=60 --monitor-only --notifications=email --notifications-level=info [email protected] [email protected] --notification-email-server=mail.example.com  --notification-email-server-port=587 [email protected]  --notification-email-server-password=abc123

[piksel]

Encryption is by default set to auto which by default means ExplicitTLS (using StartTLS) except for port 465 (which tends to be used for Implicit TLS), or if starttls=no is passed.
If you start watchtower with the --trace flag, you can see in the log exactly what URL the legacy notification handler converts your e-mail variables into. You can also just put that URL into the environment variable WATCHTOWER_NOTIFICATION_URL and set WATCHTOWER_NOTIFICATIONS to shoutrrr for complete control over the config.