CardDAV Configuration in Mozilla Thunderbird

1. Update Mozilla Thunderbird to the latest version. You need at least version 91 to access the mail.de CardDAV address book without an add-on.
2. To add the mail.de address book, first click "Address Book" in the main menu.
3. The Thunderbird address book will open. In this window, click "File -> New -> CardDAV Address Book".
4. A dialog will appear where you must enter your credentials. Enter the following:
   Username: your email address, or your custom login name if you have one
   Address: https://adressbuch.mail.de
5. After clicking "Next", the password dialog will appear.
6. Click "Sign In", and if the password is correct, a final dialog will appear allowing you to select the available address book "All mail.de Contacts".
7. After clicking "Next", your mail.de address book will be visible in Thunderbird, showing the existing contacts already stored in the mail.de address book.
8. The address book "All mail.de Contacts" is automatically synchronized between Thunderbird and our CardDAV server. By default, it syncs every 30 minutes.
   If you need to sync changes immediately, right-click on "All mail.de Contacts" and select "Synchronize".