Postfix SMTP relay server in master-slave or master-master mode for load-balancing

Our Customer have MS Exchange as mail server and using sendmail on rhel5.2 as smtp relay server.

We are planning to upgrade the systems to RHEL6.x and need 2 or multiple smtp relay servers in either master-slave or master-master (for load-balancing).

I think postfix smtp relay would be better option but not sure about master-slave or master-master smtp relay configuration.

Is smtp fall_back going to be helpful ? What would be configuration if

My mail server hostname is ----->> mail.example.com
My postfix smtp relay hostnames are ----->> relay1.example.com and relay2.example.com

Do I need to do anything at DNS side ?

Thanks in advanced for reply or update on this