I can't seem to get the QuickActionDefaultsHandler to work at all in Lightning.
Is there anybody that has got it working? Specifically, I'm using the Service Console.
Just to be clear: the "Enable Default Email Templates or the Default Handler for Email Action" is set and is pointing to the correct class.
Update from 2019: this now works in Lightning! See https://releasenotes.docs.salesforce.com/en-us/summer18/release-notes/rn_cases_email_apex_default_fields.htm
Indeed, this is not supported in Lightning. In my organization, I mainly overcame this by including the desired logic in a unified Custom HTML email template, and set it as a default for the Email action. Also, I added the recipient list, email subject, FROM, to, cc and bcc fields as formulas in predefined fields on that same Email action. If you'll share your class here, along with the various email templates, i'll be able to assist.