After new years eve I started to notice that all mails to my Kindle suddenly required me to manually accept every delivery by a two-factor authentication by mail.
To wrestle this I decided to change our email delivery service to a self hosted mailcow setup. This way we can automate the required authentication.
But as soon as I did switch to the new email service the two-factor-requirement was removed for my @kindle.com. Well, it solved the problem but made it impossible for me to automate the authentication.