AT&T have made some recent changes regarding the password. Thet now require you to use a 'secure mail key' instead of the normal password. Please read info: https://www.att.com/support/article/email-support/KM1240308/ After creating a 'Secure mail key' as advised. Access the stored password in Thunderbird and remove it. Menu app icon > Privacy & Security In 'Passwords' section click on 'Saved PAsswords' button Click on 'Show passwords' button You may be prompted to enter your computer User Account password to get access. Select the AT&T lines and clickon 'Remove' click on 'Close' Restart Thunderbird and at the prompt for password enter the 'secure mail key' you created and select the checkbox for password manager to remember password.