posteo is an independent German mail provider, focused on renewability, security, privacy and easy usage.
CalDAV/CardDAV URL: https://posteo.de:8443/
User name: your posteo email address
Password: your posteo password
No CardDAV contact group support. Choose "per-contact categories" as contact group method to minimize problems.
Only DAVdroid 18.104.22.168 and below: After creating the account, go to DAVdroid / <Posteo account> / Account Settings and disable "Use RFC6868 for VCards".
There are known server problems with contact photos, recurring events and reminders. We have already reported those problems to Posteo.