Symantec Mail gateway Load balancing through F5 LTM and Protocol security Bind for SMTP .

Being that you have more than one device, the first question I would ask is why are you trying to put the devices behind the F5. MX priority records can create the necessary redundancy. While placing the F5 in front of the devices will give you load balancing, unless you setup a routed environment where you can disable SNAT, you will lose IP context to external sources limiting your ability to control inbound flow on the gateways.