Confirmation Pop-up is Not Displayed When Sending an Email from a Case in Salesforce

To display a confirmation pop-up when sending an email from a Case, you need to switch to Salesforce Classic.

Email Sending Behaviour in Salesforce Classic

If you are using Lightning Experience, click the user icon in the upper-right corner of the screen and then click [Switch to Salesforce Classic] to switch.

1. Click the [Cases] tab.

2. Click any case record.

3. Click [Send an Email] displayed in the [Activity History] related list.

4. Click [Send].

5. The "Are you sure?" pop-up will be displayed.

Lightning Experience: Current Limitation and Feature Request

If you are using Lightning Experience, the confirmation pop-up when sending an email from a Case is not currently available. A feature request has been submitted to Salesforce Product for this functionality. You can vote for this idea on the Salesforce Ideas page.