AT&T email credentials

I am a previous AT&T internet customer with yahoo email address controlled by AT&T. I am now using a non-yahoo email address and another internet provider. I am having much difficulty trying to delete my yahoo email account/address because, I am told by yahoo customer service, that AT&T controls the credentials to my yahoo email account and AT&T must release the control so Yahoo can delete the account. Unable to reach the right department at AT&T to get assistance. HELP