NO_MASS_MAIL_PERMISSION, Single email is not enabled for your organization or profile
Sending e-mail with new comment failed: SendEmail failed. First exception on row 0; first error: NO_MASS_MAIL_PERMISSION, Single email is not enabled for your organization or profile. Single email must be enabled for you to use this feature.: 
By default, Salesforce disables mail delivery in sandboxes to prevent unintended emails from being sent. In order to send email in a sandbox, the Access Level must be changed to All Email.
Navigate to Setup > Email Administration > Deliverability to update the Access Level to All Email.
This behavior is by design.
Email to Case Premium will display a warning in the Setup Wizard and Settings pages to alert administrators when the Access Level will prevent emails from being sent.