I use Virtualmin with Postfix and this annoying error message kept popping up:

Your Postfix configuration is missing the system's mail hostname

It was easy to fix and get rid of this error message. All I had to do was edit the Postfix configuration file located at /etc/postfix/main.cf

I edited the main.cf file and made sure that my hostname was added to "mydestination" line.

If you do this fix, you may need to restart postfix to see any effect.