Problems getting email notifications with smtp to work.

[FabianZeuner]

I have problems getting email notifications working with Penpot.
But since one can only add team members by invitation this is a critical problem for me.
I'm running Penpot via docker image on a hosted server. And configured SMTP as described in the documentation

I go to project -> members -> Invite Team member, type in an address and click the send button.

1. Penpot frontend always responds with the green email successfully send notification regardless if there is an error with SMTP or not.
2. docker container logs penpot_penpot-backend_1 says: "[2022-01-03 08:53:28.440] [worker-73] E app.worker - hint="unhandled exception on task" id="9d8407f0-6c72-11ec-80c5-af28f4161d15" com.sun.mail.util.MailConnectException: Couldn't connect to host, port: localhost, 587; timeout 30000 at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:2210) ~[jakarta.mail-2.0.1.jar:2.0.1] ...

I tried multiple local (mailhog and samotop) and two of my external SMTP servers. Some with TLS some with SSL, none worked.
I also tested all SMTP servers that I tried with an PHP script which worked fine.

Since I have not found some issues regarding SMTP on Penpot GitHub, this does not seem to be a general issue, but I'm not sure what else to try.

Is there something special related to the docker container?