It may help if you share your server configuration.

outbound server settings: smtp.mail.att.net port 465 authentication method: Normal password connection security: ssl/tls

I'm no ATT specialist, but could it be you need an APP password or Oauth setting? I found this text at att.com/support: "Learn about secure mail keys and why you’ll need one to access your AT&T email from a desktop program or email app without Open Authentication (OAuth)."

Thanks, Yes, the secure mail key was the answer and did correct the issue, however; the authentication setting did not work unless 'normal password' was selected in server settings. Also, I had to input the secure mail key several times before it finally saved it as selected as it continued to ask for the password. To find the secure mail key, had a lot of navigating on the AT+T site.