How to Track Salesforce HTML Emails?

How To Track/Enable HTML Email Tracking For Salesforce Users?

Need to enable “Email Tracking (checkbox)” under Activity Settings

Go to Setup | Build | Customize | Activities | Activity Settings | Enable Email Tracking (checkbox)

HTML emails are sent as HTML File

Refer salesforce knowledge Articles: https://help.salesforce.com/articleView?id=000006731&language=en_US&type=1