You must determine whether you have an email account that is merged or not. It is possible to have a merging email account in the following scenarios :
If you do log in with your account at ATT using your Yahoo email address, you will be able to sign in with your Yahoo email
If you use ATT Login credentials to sign in to your Yahoo Mail account
If you're able to login using either your Yahoo email address or your the ATT Mail address
If you do get emails as well as messages, it is likely that they are from your Yahoo or ATT Email addresses
If you are using your same password on both accounts, you will be able to access both of them.
If you reset the password on one but you change the password for both accounts.
If you try to open Yahoo however, it will redirect you to the 'My ATT Page.'
Read More here:
https://blossary.com/Blossary:How_to_separate_yahoo_email_from_AT_T__twgid1651311813405552
