Oops, made a mistake with that last comment.
Adding "Static DHCP leases" doesnt give desired effect so the "/etc/hosts" file is the place to be.
But as a safety precaution, I have added all my static IP's as "Static DHCP lease" just in case.
And that way, I have records of all the MAC's.
EDIT: I noticed you mentioned mailserver ... you cant add MX DNS records with the "/etc/hosts" file.
For that, have a look here at the bottom of my posting: