The error message means the sender address is not allowed. Maybe because it does not appear in the mail server’s allowed senders configuration.
Huh, I found my error, thanks to your input, @pfaffman.
The ‘address’ in the error message relates to the sender address. I was focused on the IP address, so I was misled.
Once I checked the notification address (DISCOURSE_NOTIFICATION_EMAIL
in the containers/app.yml
), I realized my mail server was missing an entry for that sender: adding the alias solved the issue.