DHCP-Alert email with mac-address

How would I get the mac-address of a unknown DHCP server to show up in an email. I can get the interface and IP address with $interface and $address - but $mac-address does not work as previously mentioned on the forum.

:local sysname [/system identity get name];
/tool e-mail send to=name@domain.com from=router@domain.com subject=“DHCP Server Detected” body=“An unknown DHCP server was detected on $interface of $sysname with a ip address of $address”

Look to this webpages below for the working script

http://www.wirelessinfo.be/index.php/mikrotik/pages/dhcp-alert

I have just enabled the IP → DHCP Server → Alerts
It sends me an email even without typing anything in the ‘On Alert’ field and the email contains the mac address of the unknown server.