General, (Can't find in FAQ)

[Jonathan D. Poole]

I was hopeing some of you guys could help me understand where to allow
only say, 192.168.0.1/21 to be sending through my qmail server.  I can't
seem to find the docs for this, in specifically to my situation. 

Any help, or pointing to a document, or url, or anything would be
appreciated.

Thank You
Jonathan D. Poole