The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Authentication required

I got the following error while sending email using asp.net c#.  You should check the authentication correctly configured or not in Network Credential. If then it is proper it should be blocked by the yahoo server. 

Solution:

You need to login into your account,

Go to Account info-> Account Security->

Turn on “Allow apps that use less secure sing in”

OR

Click here: https://login.yahoo.com/account/security#other-apps

Turn on “Allow apps that use less secure sing in”