Email Load Balancing
You can load balance by sharing load between two or more email servers. Most email security proxy servers in the form of software do not usually offer a load balancing solution. However you can still achieve a simple load balancing solution by using one software server for incoming email and another to process outgoing email. If incoming is more heavily used then a more powerful machine can be used for incoming emails. Also you would need to configure higher priority MX records for incoming than outgoing if this was the case. Then configure the local mail server to send outgoing email to the outgoing software proxy server.
You can also use DNS round robin in order to send email to both email servers. The problem however with DNS round robin is if one of the email servers went down, it still sends email to this, cannot detect it is down and cannot use clever techniques such as sending more mail to the more powerful server.
Ideally for a real and effective load balancing solution you can use proprietary load balancing software like Microsoft load balancing server. The load balancing software will allocate messages based on the processing loads of the individual servers rather than a simple sequential technique such as using MX records. For an even better and true load balancing solution, you would need a dedicated hardware load balancer like the F5 or Barracuda load balancer.
Wikipedia's guide to Load Balancing