How can I send an email using the Rules module to a user specified via Entity Reference in a Content Type?
After some trial and error, I found how to reference the field.

Within the Conditions section of the rule, I needed to:


Add "entity has field"
Enter "node" within the Data Selector box
Select the entity reference field within the Value dropdown


I was then able to refer to the field required, with the mail parameter appended, directly from a standard Send Mail action (i.e. [node:MY-REFERENCED-FIELD:mail]).