501 5.5.4 Sie dürfen von dieser Adresse nicht senden

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.

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