Filtering mail from outside users only

[Vicki Brown]

I am reading http://www.postfix.org/FILTER_README.html 
<http://www.postfix.org/FILTER_README.html>, the section on "Filtering mail 
from outside users only" and I am befuddled.

Am I correctly understanding that this recipe requires three 
    smtp  ... 
entries in master.cf?

Where can I find documentation for 1.2.3.4:smtp and 1.2.3.5:smtp?

- Vicki