GMX is a German Web portal which provides an email service with address books and calendars.
Two separate DAVdroid accounts are required (one for CalDAV, another for CardDAV) because there are no SRV records.
- either use "Login with email address", or
- base URL:
For CardDAV, use this base URL:
User name: your GMX email address
Password: your GMX 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.