The message says:
Dear AT&T Yahoo! Customer:
We noticed that you are accessing email using non-secure settings in your email software.
We would like to ensure that your AT&T Yahoo! Member ID, password, and email messages are transmitted securely between your mail software (such as Outlook or Outlook Express) and the AT&T Yahoo! Mail servers. In order to meet this need, please enable SSL via the instructions that are available on the Help site.
Since multiple email notifications have already been sent out about this, we request that you please make the necessary changes immediately. Remember, you need to make these changes if you want to continue to send/receive email using a mail client.
Thank you for your cooperation,
AT&T Yahoo! Customer Support

I have used the provided link AND the manual directions to resolve the issue MANY times, and it appears to be correct. However, I keep getting this message. I Outlook Express to send and receive e-mail, but use Outlook for its calendar, contacts and task list.