The Envelope icons on Directory Pages (as well as email links that spell out the recipients' address) are "mailto:" links. These are customized hyperlinks that always point to email addresses, as opposed to web address. Mailto: links will open up the designated default email application on the user's current computer. (This is why the same link might open up Outlook on one computer, and GMail on another. Those two computers are configured to have different default email applications.)
Default applications are controlled by your computer's operating system. You can set your computer to open whichever program you want when clicking on, say, an email link. This is usually adjusted in your operating system's "Settings" menu.