To Check or Update Sign-In Info in Thunderbird * Open Thunderbird * Click Thunderbird -> Account Settings * (alternatively, In the left-hand pane, right-click your email account and select Account Settings) * Go to Server Settings on the left There you'll see: * Server name (this is usually something like. pop.mail.yahoo.com) * Username — this should be your FULL email address * Authentication method — should be set to OAuth2 (if available) or Normal Password if using an app password