I am getting the popup error message in Thunderbird of "Authentication failure when connecting to server imap.mail.yahoo.com". The issue just started on one of my two Yahoo mail accounts with no change of details (either in Thunderbird or server side). I have verified that ALL settings are the same as the other Yahoo account which is working properly. Apart from the really annoying popup, Thunderbird still seems to function ok, emails send & receive etc. Current settings in Thunderbird are: User Name: FULL Yahoo email address Server Type: IMAP Mail Server Server Name: imap.mail.yahoo.com Port: 993 Connection security: SSL/TLS Authentication method: OAuth2 Operating System: Windows 7 (Windows 10 on alternate (New) PC) Things I have tried so far: Changed password several times Enabled and disabled the 'Allow apps that use less secure sign-in' on server side (no difference) Changed 'authentication method' from 'Normal password' to 'Encrypted password' to 'OAuth2' Changed IMAP server to Yahoo's alternate one: imap.mail.att.net (no difference, reverted back to imap.mail.yahoo.com) Installed a new installation of Thunderbird on a different (New) PC - same error message I love Thunderbird but am at a total loss as to how to resolve this issue, any assistance would be greatly appreciated.