We do not use a mail system (unless you installed CSF) so I am not too terribly sure why this would be asked here. If you look at the message it has produced, it looks quite clear as to why the message has occurred..
Sender address rejected: Domain not found
You could install sendmail with
apt-get install sendmail and then within your /etc/aliases file add in the bottom that root@ mail be directed to
Pretty sure the default
/etc/aliases file looks like:
# See man 5 aliases for format
If you have installed a mail server on your server within your own needs, you are on your own. My days with email administration are over as it is a frustrating battle with security and stability.