To answer your query, ESA can be setup in a cluster but not in HA as we can in case of firewalls such as ASA.
You can read a little more about ESA clustering in the article below. 'Clustering' right now on the ESA basically just means sharing the configuration across multiple appliances. Unfortunately, there's no way to setup a true failover/HA configuration on the device as it stands right now. Aside from the ESA, however, you can always setup multiple MX records and/or load balancing to try and perform the requested functions.