TB supports OAuth2 for IMAP accounts for a few mail providers, such as Gmail, Yahoo, AOL and Yandex. Support for other providers is added on a provider-by-provider basis. With AT&T, you can still use 'normal password' authentication, but in place of the account password, generate a 'secure mail key' and enter that instead. https://www.att.com/esupport/article.html#!/email-support/KM1240308?gsi=4icj1v If you have a password stored in Tools/Options/Security/Passwords/Saved passwords, click Show passwords, Edit Password, and paste the secure mail key - for both the incoming and outgoing servers.