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>