Hi Himbo,
I’m not familiar with Nullmailer but, from what you posted it seems that it logs to the syslog file.
As a troubleshooting step, I recommend watching the syslog file while triggering an email notification from within Checkmk. You can watch the syslog file with tail -f /var/log/syslog. If you find that there is a lot of unrelated information being written to syslog during your test, you can filter the output with grep: tail -f /var/log/syslog | grep -Fi nullmailer. Depending on your system configuration, you may need to preface the commands with sudo to request elevated permissions.
If there really is nothing shown in the logs then Checkmk is likely failing to pass the message to Nullmailer correctly. If there is something in the logs then the log messages should give a better indication of what is happening.
Hope this helps,
Jason