Need help setting up linux VPS mail server to send to inbox

I have set up my linux VPS already(Centos OS). Postfix is installed. Domain and rdns setup. However, emails are going to spam and I don't know why. I think it is something I am doing wrong in the configuration. I need somebody who is experienced in doing this who can make corrections to my VPS so it will send to inbox at most of the big email isps(gmail, hotmail, yahoo). You will also need to teach me how to do this so I can set up VPS boxes without problems in the future.

I would use exim instead of postfix as its easier to set up rotating IPs on it.