Digitally sign your email with DKIM (DomainKeys Identified Mail) and help prevent email spoofing on your side. DKIM adds a private key which stays in your server and a public key that stays in your DNS record. The outgoing email header that you send gets encrypted and the recipient servers then retrieve the public key to decrypt the incoming headers and verify that the messages really comes from your domain and was not altered while it is on transit.
I keep on seeing a few multi-site Wordpress installation using a sub directory but a few have a multi-site using a subdomain. Firing up curiosity, I tried installing a one using subdomains instead.
This guide will teach you the easy way to do things using virtualmin.
Virtualmin is configured to setup DNS zones but this system is not setup to use itself as a DNS server
Failed to save enabled features : Virtualmin is configured to setup DNS zones, but this system is not setup to use itself as a DNS server. Either add 127.0.0.1 to the list of DNS servers, or turn off the BIND feature on the module config page.
This is a generic error telling the website administrator that the local DNS entries are being checked using external methods. Most of the time, this wont be a real issues but it causes a performance spike which depends highly on where the DNS entries are being searched.