[Exim] Secondary Server

My mail server is currently set up as a DNS server aswell as a mail server.
I would like to set it up as a secondary server for a specific domain. Where
do I start and what do I do?

Thanks, the forum is a great help

Gareth Otte
Technical Assistant
ELCB Information Services
Phone:  (043) 7220757
Fax:      (043) 7220846
email:     gotte@???