This action can be simply done by using DNS Round-Robin, that's when you configure in a DNS more than one A record for the same name. So at each query to resolve this name, the DNS server will respond with a different IP address, cycling into the set of A records provided for this unique name.
But in this scenario, there are a lot of potential problems :
These are solutions that are just a Web Access layer that you can put over any existing mail server.
With its plugin framework, it is very easy to be expanded to your needs.