Recently Yahoo and Gmail ask users if they wish to block less secure apps to secure their account.
By denying use of less secure app in Yahoo Mail and Gmail account, older mail software such as Outlook Express, Outlook Desktop, Windows Live Mail, Windows Mail and Apple Mail on Mac OS 10.9 or lower will not be able to communicate with Yahoo or Gmail. This means no more sending or receiving email!
This issue also affects older Smart Phones and Tablets.
To resolve the problem:
Download provided apps from Yahoo or Gmail and configure your email on new apps
Login to your Yahoo Mail or Gmail, go to Account Security setting and select “Allow apps that use less secure sign in” for Yahoo Mail and select “Turn On” for “Less secure apps” for Gmail.