Yahoo have changed what they allow for the 'Authentication Method'. * https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo#w_important-changes-to-authentication-method-notice bellsouth is a AT&T domain on yahoo server, so two possible methods. Check server settings. Bellsouth.net (AT&T) '''IMAP''' Server name: '''imap.mail.yahoo.com''' IMAP port: '''993''' Connection security '''SSL / TLS''' Authentication Method:''' OAuth2''' IMAP username Your full Bellsouth.net email address IMAP password Your Bellsouth.net password Bellsouth.net (AT&T) SMTP Server name: smtp.mail.yahoo.com SMTP port 465 Connection security SSL / TLS Authentication Method: OAuth2 SMTP username Your full email address SMTP password Your Bellsouth.net password when you restart after checking settings, you will get prompted to logon to allow Thundrebird access and then a token is created and stored in Thunderbird. cookies must be enabled in order for this to occur - some info at above link. '''Alternative:''' AT&T say there has been a change in Authentication Method. So if you cannot / do not want to set up imap using OAuth or do not want to use the server settings as above then you need to create a 'Secure Mail Key' as per instructions at link above. Then use that 'Secure Mail Key' instead of the normal password in Thunderbird. * https://www.att.com/support/article/email-support/KM1240308/ Remove stored password in Thunderbird Restart Thunderbird then enter that 'Secure Mail Key' at the password prompt.