Email Authentication

I've seen a few messages on this board that include
email addresses with a little note saying that if you
send email to that address you will have to verify
your own email address before it will be accepted.
I am interested in adding that capability to my own
email system. I use Linux 7.3 with Postfix. Any
tips on how to do this?
--
Phil Mattison
Ohmikron Corp.
480-722-9595 ext. 1