To enable open and link tracking on all messages being sent via Easy-SMTP, simply navigate to the upper right hand corner, and click on your User Settings. This is represented by your Account Key along with a cog wheel icon. Once you've clicked here you'll be taking to your User Settings page. Navigate to the section titled Easy-SMTP and check Enable Open and Click Tracking.
When this setting is turned off open and link tracking can be enabled per-message by using the "X-Tracking" header in SMTP submissions or setting the "Tracking" option to "true" in API submissions. Click below to see where to locate this feature inside the user interface.