Open Authentication, or OAuth2, is the preferred method of authentication if your account connects to the Yahoo servers such as imap.mail.yahoo.com and smtp.mail.yahoo.com. But if the account connects through the AT&T servers such as inbound.att.net or imap.mail.att.net and outbound.att.net, the authentication should be 'normal password' and a secure mail key, created on the AT&T site, should be used in TB instead of the account password. https://www.att.com/support/article/dsl-high-speed/KM1010523/ Open Tools/Account Settings and see which servers are entered and the authentication method.