Bouncing mail because of a non-existing domain

Hello all, I have standard SuSE8.1 setup at home and Kmail can receive mails fine, but it can't send mail. IKmail is using Postfix. My smtp setup in Kmail is in the form : "smtp.isp_mail_address" When I use kmail to send I keep getting the following message... "Sending failed .... Sender address rejected: Domain not found" I pinged "smtp.isp_mail_address" and added "smtp.isp_mail_address" plus the returned IP address to /etc/hosts file but that didn't change anything. What gives? I surely don't need have a registered domain to use kmail. Any ideas would be appreciated. Thanks in advance James