Mail Server Machine Name
ie. lixx-xxx.members.linode.com instead of mail.mydomain.com
That way your domain (mydomain.com) and all others can be setup as a virtual domains.
Though I prefer having it set to "mx1.betadome.net" on mine (increasing the number when I get around to set up more servers), just to identify who the mail service provider is (betadome.net, which is me).
What exactly is set up as the mail server's machine name is entirely up to you, though, as long as the server host address you're using actually resolves to your Linode IP (or at least one of them, if you have more than one).