Hello, my name is ***** ***** I will do my best to assist you. Give me a few minutes to look over your question and I will respond shortly.
Are you using Windows Live Mail or another e-mail client and getting this message? Or are you signing into the Yahoo! Mail website?
You may try to logon on to your Yahoo! account through Chrome with this link: https://login.yahoo.com/account/security#less-secure-apps and select Allow apps that use less secure sign in.
This might be why your e-mail client cannot connect.