Hi everyone,
Did anyone find a solution to this?
I’ve setup postfix on my docker host for outgoing emails. It works fine when I send emails from ssh via the mail command, but discourse docker is failing with the same error as the OP got. I’ve used a hint from this thread: How to set SMTP config to use localhost? and thanks to that I got rid of authentication error from discourse docker (I do not provide any user/password to discourse docker to connect to my email server) , but now getting ‘sending mail failed’. Any ideas?
I had no idea that setting up an email server would be the hardest part in my discourse journey… 
The discourse install is just flawless and so well thought unlike any linux email server setup.
Thanks in advance!