Windows 8 sets its new Mail app as the default email client. This is annoying when you click an email (MailTo) link on a webpage and youâre bounced into the new UI. Hereâs how to change it to a different desktop email program like Outlook. Changing the default client from the Mail app to something else is similar to making PDFs and Photos open on the Desktop, or preventing videos and music files from opening in Windows 8 apps.