POstfix with multiple domains

[trav04]

Hello, i am running Fedora Core 4, Perfect Setup with ISPConfig installed,
Problem :
Dont know how to setup with multiple domain names.
Also im trying to a full dns. I am currently using Afarid.org for support.

Solutions:
-- Painreliever for headache
-- sleep
-- help

I have been trying to figure this out, found many useful sites just dont know what is going wrong. The server is sending mail from the localhost.localdomain
i want to change it to my domain names.
I can give you access if need be or logs. Thanks in Advance (TIA)