Hello. I have setup a Debian Lenny server with multiple client domians and it works fine. But since I use Google Apps (with 500 mail limit per day) I need to setup some sort of mechanism to send mail through my server to get past the limit. I use e.g. Swiftmailer script to send mails through PHP and wants to keep doing that. I think setting up Exim or Postfix would get the job done. But I don't quite know where to beginning can you point me to some guides or tell me where to start? Be sure to notice that I will never recieve mail on this server. Only send mail.
Chapter 12 on http://www.howtoforge.com/perfect-server-debian-lenny-ispconfig2-p4 should get you going. Is your server in a data center, or is it hosted on a dynamic IP?