I get an error mail from the server, about an email that could not be sent.
The error mail is sent successfully by ssmtp.
The mail that failed to get delivered, is failed because it's sent by the cron service to:
andre@odin2 (no such email address exist)
Technical details of permanent failure:
DNS Error: Domain name not found
I've tried to force all CRON email to go to correct email address to crontab like this
MAILTO="firstname.lastname@example.org" (That did not help.)
(in case you suggest a mail server, Procmail is installed, but not configured.)
To me, it seems that the user's default email address needs to be entered somewhere.