Yahoo says the Mail App that came with my Nokia Lumia 720 is not secure enough, that I should get the most secure Mobile Mail App from Google App Store
As stated in a previous question about the security of the Mail app, the app is only secure if you are using a secure mail server. The message given to you from Yahoo might be entailing that windows phones cannot use secure protocols for sending emails. The mail app is secure as I have been using it for 5 years and have had no security issues. The Mail app on Windows phone does use SMTP(Simple Mail Transfer Protocol).