what prabhat said is correct, but if you use freepops ( GPL, Freeware,
http://www.freepops.org ) you can use that software to convert yahoo to pop3 account
basically freepops is a pop3 to http application proxy that converts pop3 requests from kmail to browser actions of downloading email
however this will work only for yahoo there is no plugin for rediff yet. every website requires a plugin created in LUA to be able to download the email from that website into outlook / kmail / whatever pop3 client you are going to use.