Send Email Action in Salesforce Lightning

Introduction

Lightning email action allows emails to be sent anywhere in Salesforce. Allows great customization, such as adding and subtracting fields or specifying predefined values. Action also supports all items with a enabled function, and can be requested anywhere in the Lightning Experience.

Send email action is available for all objects. But if you want implement this action on case object then first you have to enable email to case functionality in your org.

Steps to Create a Send Email Action

From Setup, click the Object Manager tab. Select object on which you want to create this action, then open Buttons, Links, and Actions setup page.
Click New Action.
For Action Type, select Send Email.
For the Standard Label field, select the label for this action. When you add an action to the page layout, this label is displayed in the action bar.

The Name field fills automatically.
In the Definitions field, describe what this quick action is doing. The description appears on the action information page and in the Button, Links, and Actions list on the page. Description is not visible to your users on user interface.
Send email setup information page
Click Save.

Don't forget to check out: What are Salesforce Lightning Web Components?

Steps to Customize Email Layout

After click save the email layout page opens, where you can specify the fields displayed in the email action layout.

Organize email fields the way you want for the users
From the Email Message pallet, drag the required fields into the email layout. To remove fields, drag them from the email structure to the Email Message Fields field.
To Address, from picklist, Title, and HTML Body fields are added automatically. For information on the fields, see the Available Fields in Email Send Quick Steps
Add fields to an email structure

To make the field readable or necessary, hover over the field and click the Wrench icon and specify field features.
Playground properties menu
To save the field properties, click OK.
Click Save.

Steps to Set Predefined Values for Email Action

Select the object in which the email action is created.
Open the Send Email action you want to modify.
Locate Predefined Field Values section, click New.
With a Domain Name, select the field you want to set the default value for.
Use the formula editor to merge your selected field with a Salesforce record.
Click Save.