Check and, if needed, Update your Sign-In Info in Thunderbird 1 Open Thunderbird 2 In the left-hand pane, right-click your email account and select Settings 3 Go to Server Settings There you'll see: • Server name (e.g. 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 Go to your inbox. Right click on Inbox & go to Properties -> Repair Folder. Then Restart Thunderbird