How to make Notifier Pro for Gmail your default mail app
Since Yosemite, Apple is not allowing the sandboxed application to change the default application handler, so we can’t make it within the application itself.
But the good news is that Notifier Pro for Gmail is able to answer to the mailto: URLs, and here is how you can proceed to make the change:
1. Open the Mail.app (the one installed by default on your system).
2. On the top menu, Mail -> preferences
3. in the General tab, the first option: Default email reader . Change this to ‘Notifier Pro for Gmail’, located in your Application’s folder.
that’s it, now next time you click on a mailto: link in a browser, it should open your Gmail compose window.