Can the smtproutes command be used for load balancing?
The smtproutes command in the CLI, or the Network->SMTP Routes page on the GUI, allows you to redirect all email for a particular domain to a specific mail exchange (MX) host, without rewriting the envelope recipient.
Multiple destination hosts can be specified for a particular domain name. If you specify multiple routes for a particular domain name, the Cisco ESA will cycle through the destination hosts you specify in a "round-robin" fashion.
There is no load balancing performed by the Cisco ESA on smtproutes.
If you need to load balance your internal mail exchange hosts, you will need to configure your Cisco ESA to redirect mail for your domain(s) to your already-configured load balancer. The load balancer is responsible for balancing the load between each mail exchange host for which it is configured.