If you are just using the normal password you would use to access webmail then it is possible outlook does not think it is secure. Options: Either: Turn on Two-Step Verification, then generate an app password which you would enter in Thunderbird instead of the normal password. https://support.microsoft.com/en-us/account-billing/using-app-passwords-with-apps-that-don-t-support-two-step-verification-5896ed9b-4263-e681-128a-a6f2979a7944 OR: Try using different server setting: https://support.microsoft.com/en-us/office/pop-imap-and-smtp-settings-8361e398-8af4-4e97-b147-6c6c4ac95353 * IMAP Server name: outlook.office365.com * Port: 993 * Connection Security: SSL/TLS * Authentication Method: OAuth2 * User name: Normal Password * SMTP server name: smtp.office365.com * Port: 587 * Connection Security: STARTTLS * Authentication Method: OAuth2 * User name: Normal Password Make sure you have set up Thunderbird to allow cookies. * Menu icon > Preferences > Privacy & Security Web content: * select 'Accept cookies from sites' Exit Thunderbird Wait a few moments for background processes to complete then restart Thunderbird. You will get a prompt to enter email address and normal password to allow Thunderbird access. Then an Oauth2 token is created and stored in the same passwords manager. It will be used for future access to server.