Phone won't add email account due to incorrect password

Make sure you can login to the web portal here using your password?

https://more.att.com/login/sbcglobal-email

I believe you may have to choose Yahoo as the Mail Type when adding the email account since it is being managed by Yahoo Mail.

Is this an iCloud email address or a third party email?

Try signing into the web portal for the email. For a third party provider, many will have another verification step when adding to an email client. Depending on who the provider is, we may be able to offer additional help.