Temporary authentication failure

May I know how you fixed the issue? I am facing similar issue while trying to add email to Thunderbird on Android.

Thank you!

So I am using two-factor authentication (2FA), if you are not, then this might not be the solution for you.

Here is how I got it to work (again):

1. Go to "All settings..." on app.mailbox.org:

2. Click on "Security", expand "Email app-passwords" (important - not "App passwords"), delete the existing one (this might not have been necessary), enter a new password name (here it would make sense to type "Thunderbird" in your case), check both boxes (IMAP/SMTP) and finally click "Add new password":

3. In the email client, for the username you have to use the same as when you log in to webmail, and for the password, the newly created "email app password" (that will be shown to you after the previous step).

Thanks a lot! I had 2fa activated and your steps helped.

Thank you Claus. I have been searching for days and had not seen this setting. This resolved my problem. Tschüss!

I spent a ridiculous amount of time looking for the setting for application passwords, until I finally found it: Settings -> mailbox.org -> My account -> Email app-passwords. I do not understand why it isn't under "Application Passwords" in the settings menu.