Can receive but can’t send email using Windows Live Mail.

Error code 530 says smtp server rejects my email user name. After two hours on the phone with

 ATT tech support, they attribute problem to my mail settings.  A diagnostic says the problem is

APP incompatibility.  Everything was working until last Saturday. 

Complete error message is as follows:

"The message could not be sent because the server rejected the sender's email address.

Sender's email address was . . . .

Server Error: 530

Server Response: 530 authentication required - for help go to . . .

Server: 'smtp.att.yahoo.com'

Windows Live Mail Error ID: Ox800CCC78

Prostocol: SMTP

Port: 25

(Last line not readable)

<Windows 7/IE8>