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.

once you have an account setup in mail.app,

goto settings > general

pick your default email client from the list.
also, change the ‘check for new mail’ setting to ‘manual’ so that your mail.app doesnt fetch mail all the time.

you can now close mail.app and forget about it.
from this point on every mailto: link you click will open the compose dialogue of your chosen email client :)
it will also work for the ‘share by email’ feature in safari!