[CentOS] [CORRECTED] Postfix SMTP authentication on remote relay serve

Sorry about the previous post, I see the subject had a typo. So here's the correct one :)

Does anyone know how to configure Postfix to use SMTP authentication on a relay host?

i.e. I have a Linux server, running PostFix on an ADSL line with a dynamic IP. Thus I can't just send out emails from this server but need to relay it via our ISP's SMTP server, which is very unreliable. Now I want to relay the email via another SMTP, running cPanel server where a user account with the same domain has already been setup, but I got no idea how to get it to work on SMTP authenticiaion.

Any suggestions?