The message could not be sent because the server rejected the sender's e-mail...

...address. The sender's e-mail add The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'reddone@comcast.net'. Subject 'Read: New address', Account: 'COMCAST Mail', Server: 'smtp.att.yahoo.com', Protocol: SMTP, Server Response: '530 authentication required - for help go to http://help.yahoo.com/help/us/sbc/dsl/mail/pop/pop-11.html', Port: 465, Secure(SSL): Yes, Server Error: 530, Error Number: 0x800CCC78

Yahoo has upgraded its servers. You will need to make a small change to your Yahoo! Mail Incoming Mail and outgoing mail servers address by adding the word "plus" as below.
* Incoming mail server (pop3): plus.pop.mail.yahoo.com
* Outgoing mail server (SMPT) : plus.smtp.mail.yahoo.com
You need to have yahoo mail plus.
You should Check your setting with the links below.
http://help.yahoo.com/l/us/yahoo/mail/original/mailplus/pop/pop-35.html
http://help.yahoo.com/l/us/yahoo/mail/original/mailplus/pop/pop-14.html