1. Ensure “Enable Case Feed Actions and Feed Items” is enabled under support settings. To modify Support settings : Go | Setup | Customize | Cases | Support Settings.
2. If you do not find the setting “Enable Case Feed Actions and Feed Items” under support settings, make sure “Feed Tracking” is enabled for case object. To enable Feed Tracking: Go | Setup | Customize | Chatter | Feed Tracking | Select Case object | Enable Feed Tracking.
3. The email Deliverability settings | Access level must be set to ‘All Email’ or this component will not render.
4. Please note that if you not at all find the Email Publisher action to add on Case page layout, it is because “Email to Case” is not enabled for the organization. To Enable: Go | Setup | Customize | Cases | Email to Case.