I am able to receive emails, but no longer can send emails.  The following error comes up:

The message could not be sent because the server rejected the sender's email address. The sender's email address was '(omitted for this post)'.

Subject 'test'
Server Error: 553
Server Response: 553 From address not verified - see http://help.yahoo.com/l/us/yahoo/mail/original/manage/sendfrom-07.html
Server: 'outbound.att.net'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 465
Secure(SSL): Yes