Looking for instructions on how to configure home server as a restricted relay host

[Reid Thompson]

What I would like to do:
Configure my home postfix server (ubuntu) to:
  send email from local user accounts
  accept external (through my cable modem) smtp requests/relay mail for 
only authorized senders
     I.E. when I'm using a public internet connection, i'd like to have 
my smtp requests go through my home server

Could someone point me to a website describing how to configure this?

thanks,
reid