Using prior written troubleshooting advice from Thunderbird users, I learned that I must have a secure mail key.  Following prompts from the ATT website I established a secure mail key and then installed the secure email key, all 16 alpha-letter characters, in the password section of my Thunderbird email program.  I now get a new message, which I repeat below:
  "The server does not support the selected authentication method. Please change the 'Authentication method' in the 'Account Settings | Server settings'."
 I contacted ATT tech support, first level, and changed the outgoing server to smtp.mail.att.net.    The tech seemed amazed that I could not touch the incoming server setting.  
 The second level tech at ATT says I have to delete the account in Thunderbird and then re-establish the account in Thunderbird, using a new secure mail key obtained from ATT.  
 Question:  is there a simpler solution that re-doing the account as the second level tech says?  What setting must I alter in Thunderbird to comply with the new message and get contact with the ATT mail servers?  What am I doing wrong?  Thank you in advance.

In late April 2021, my Thunderbird client stopped getting access to my email account at sbcglobal.net. My understanding is that ATT now controls his email server and address. Oddly, I could get full access through the Yahoo mail website to the account and both send and receive emails thru the web service. Using prior written troubleshooting advice from Thunderbird users, I learned that I must have a secure mail key. Following prompts from the ATT website I established a secure mail key and then installed the secure email key, all 16 alpha-letter characters, in the password section of my Thunderbird email program. I now get a new message, which I repeat below: "The server does not support the selected authentication method. Please change the 'Authentication method' in the 'Account Settings | Server settings'." I contacted ATT tech support, first level, and changed the outgoing server to smtp.mail.att.net. The tech seemed amazed that I could not touch the incoming server setting. The second level tech at ATT says I have to delete the account in Thunderbird and then re-establish the account in Thunderbird, using a new secure mail key obtained from ATT. Question: is there a simpler solution that re-doing the account as the second level tech says? What setting must I alter in Thunderbird to comply with the new message and get contact with the ATT mail servers? What am I doing wrong? Thank you in advance.