How come when I select an email in Pipeline, an email client pops-up?

Which email app opens when you click an email address is dependent on your operating system settings. For instructions on how to change which email app is opened please see Change default programs in Windows or Change the default web browser or email app on Mac. 

If you are attempting to have links open in an external email client (like Outlook), but they are still opening in the browser you may also have to remove your email protocol handler setting (in Chrome or Edge). The option is located under Settings -> Privacy and security -> Site settings -> Additional permissions -> Protocol handlers