Email Password Change incomplete

Hi 

My email service is att.net via yahoo, since when I had a landline. The password change process routes to sites that start with uverse3. I changed my password -- apparently -- and when I login to att.net site via a browser I MUST use the new password (that is good). However when I get email from iphone or mac mail app I must use the old password to make it work, the new password does not work. The old password should not work but it does. 

I worked with 10 AT&T agents all of whom seemed to be oursourced to other companies...from ConnecTec to AT&T customer service suppliers. None had any jurisdiction to even take ownership of the trouble. A very nice agent was told by her manager not to continue providing warm transfers as it was messing up their metrics. The problem remains and is a security risk in that I am unable to make the old password stop working. 

How can this issue be fixed.