How to setup Multiple Mail Servers

I want to setup a server that would accomodate 200,000 users with an alloted 5mb of email messages each. At the moment our mail server is running in a server using both smtp and pop3 on that machine(sendmail 8.11.6/8.11.6, Qpopper 4.0.3, Red Hat Linux 7.0). Previously, I have setup 2 mail server and declared that on my dns server. When mail server A dies, mail server B accepts the mail. The preference is 10 for mail server A and 20 for mail server B. That’s for redundancy. But what I want is that to have multiple servers. For example for the first mail server handles the first 100,000 users, while the others handles 200,000 users. How can I do this?

thanks in advance.