Forwarding mail to ISP

I have a cable modem/router setup locally(Motorola SBG1000). My SuSE server is setup to run fetchmail to download mail from the ISP then pipe it through Postfix then to Cyrus IMAPD. This is all working fine. Now I want to be able to use Postfix to forward the mail to the ISP's SMTP server. I read how to setup the canonical hashfile to change the address to fool the ISP's server in to thinking it came from a E-Mail program such as KMail. How do I setup the MX files using resolv.conf so Postfix knows where to send the mail. Or, is this needed? NeoFax