Your ID and any AT&T email addresses you see are linked and share the same password. No matter which one you sign in with, you’ll use the same password and have access to any account or service linked to the ID.
Good to know: Make sure you use the new password to sign in to your AT&T Mail account, if your username is the same as your AT&T user ID. You should also update your password wherever you have it saved. This includes your devices, web browsers, and any apps.