I believe the information you need will be found on ATT BIS website, or on Yahoo email website, but the settings need to be made on the BIS site. It's a matter of enabling your BIS account to access your Yahoo email via SSL. I think your notice from ATT refers to the BIS site accessing your email insecurely. I poked around but didn't find anything quickly, but there are more detailed settings in creating the account on ATT's BIS site that you may not have seen. For example when I setup IMAP for GMail, I had to access other settings that were not available to me initially. When adding your account, input your email address, but not a password, and it should allow you access to a button that says "I will setup this account manually"