@MaxRink is it still possible to use the Docker image without SMTP TLS? Not sure if someone is using that?

Well, the dockerfile curretnly in use sets the environment Variable "SMTP_NOTLS" to true.
I canoOverride this at launch to false but looking at the code that interprets the ENV-Vars ( https://github.com/frab/frab/blob/master/config/initializers/action_mailer.rb#L12 ) the sole presence of this variable in the environment disables TLS-Support for good, no matter what its actually set to.

And as far as i know there is no easy way to unset an env var defined in an dockerfile so the Variable needs to be dropped from the presets within the Dockerfile to get SMTP TLS working.
You can still define it at launch if you want to drop TLS Support tho.

And i know that that isn't the real solution to this Issue (i'd propose to change the handling of the ENV_Var to actually reflect the value, not just the existance) but its a quick fix that gets that working again.