Issues Sending Email...
Site:
Here's the issue in a nutshell. At least one of the smtp.att.yahoo.com servers has incorrectly installed SSL certificates. This means when an attempt is made to verify the connection (since it is supposed to be a secure connection) the certificate check fails.
The problem is sporadic, which probably means that there is more than one smtp server behind smtp.att.yahoo.com, and at least one of them is set up correctly and the certificate checks pass.
Bottom line: ATT has to fix their server(s) to send valid certificate information.