This simply doesn't work.
I can add an email address, but if I try adding a provider, or editing options for existing provider the app hard crashes EVERY time.
This on a 27 inch iMac (2013) running el Capitan.
The result is that no recipients get added to the message generated in Mail.app, and so the message does not get sent!
Assume that the 'provider' is to setup an email provider, but notarially clear on its purpose, if if it's even necessary... The fact that NO recipients are added to out going messages at the moment suggests it IS necessary, but would like some clarity