posteo is an independent German mail provider, focused on renewability, security, privacy and easy usage.
DAVx⁵ settings
Choose "Login with email address" and enter your Posteo email address and password.
If this doesn't work, you can try:
Base URL: https://posteo.de:8443/
User name: your posteo email address
Password: your posteo password
Contact group method: vCard contact groups are not supported. Choose the method used by your other clients or "per-contact categories" to minimize potential problems.
Known problems
There are known server problems with contact photos, recurring events and reminders. We have already reported those problems to Posteo.