re :''Log in as...'' That sounds like you are talking about the small pop up window which asks for username = email address and the account password and you then click on 'Log in' in order to allow Thunderbird access to the yandex server mail account. At which point it sets up the oauth token. That is normally required if the oauth token is not stored in Thunderbird and requires you to get one. But, it looks like you clicked on 'Register' or got redirected by yandex to register as stated this info: https://yandex.com/dev/id/doc/en/register-client Yandex says: https://yandex.com/dev/id/doc/en/access ''The received token can be saved in the app and used for requests until its lifetime expires.'' So has that 'lifetime' expired ? Found lots of good info here: https://yandex.com/dev/id/doc/en/concepts/ya-oauth-intro But yandex themselves do state you will get redirected to that page as per your image if registering is required. If the password was changed then a new oauth token will be required. If the saved passwords were removed or not stored in Thunderbird then you need to set up a new oauth token. Oauth tokens are stored in the same place as all other mail account passwords. Is there any oauth token currently stored? * Menu app icon > Settings > Privacy & Security * Undre 'Passwords' section * Click on 'Saved Passwords' * click on 'Show Passwords' There should be 'oauth://account...' line for the mail account. If the line does not exist then a new oauth is required. But you do need to find out if the lifetime has expired and whether you need to register again.