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