Have you set up 2 step verification and created an app specific password ? https://help.aol.com/articles/Create-and-manage-app-password In AOL webmail, click on "Options" in the upper right, under your email address. Go to Account Info Go to Security (list on the left) Scroll all the way down to "Manage App Passwords" Generate a code for "other" by inputting "Thunderbird" Copy and save it to clipboard. then in Thunderbird when it asks for password, use that app generated password. Maybe you are using the yahoo servers ? If imap then perhaps you try using the Authentication Method: OAuth2 https://support.mozilla.org/en-US/questions/1282410