This post will show you how to change the default email client in osx.
it will let your chosen email client be used for both mailto: links and for the safari share with email feature.

the actual setting for the default email client is in mail.app

you need an account setup in mail.app in order to access the setting :|
so, firstly, if you dont yet have an account set in mail.app
open mail.app and set one up.