I am looking for building an SMTP server that could handle sending some millions emails per month.
I am building an application(PHP based) more like a social media that users would be sending messages and invites using the application's internal mail system.
This SMTP server will only be responsible sending these emails.

Currently I am thinking that the web servers will be running on Amazon Cloud while our smtp server will be based on our dedicated servers.

I would like your advice on

1) What kind of software should I use (postfix, sendmail, etc...)
2) What would be the hardware requirements for such server (CPU, RAM, Storage etc...)
3) How can I restrict that this smtp server is used only for requests send from my application? My Amazon cloud probably won't have static IP addresses expect my load balancers.
4) If the volume of emails is increased to a certain level should I use some kind of load balancing between smtp servers?

Thank you in advance.

I can't advise, but nothing in your questions is specific to CentOS-5 or even to CentOS, so your thread is moving to Social.