I have a few computers in a network with one computer installed Red Hat 9.0 as server, and other's are clients. I want to set the clients just able to send email to Yahoo mail only, how to do it?

on your red hat 9.0 box go to http://www.webmin.com this is a webbased administration tool for configureing servers wich is what were gona do sendmail and squid (a proxy server) are two of the more difucult projects for new users so we will simplify with webmin if you want to later take a manual look at the changes webmin made to the conf files thats your decicion first after installing and firing webmin go to the squid tab squid is a internet proxy serveice which means you will be able to restrice what ports and access is allowed the main key is that after specifying what ports you want open (namely what ever ports yahoo mail will require such as 80 (3128) 110 25 etc... then after that make sure that all other ports are denied then you simply deny all access to all domains except yahoo that simple now they can only do the few things on the internet you allow at the few places you allow then you simply edit the clients internet properties to use a proxy server and point them to the linux box by default squid uses port 3128 instead of 80 like microsoft proxy did