GMX is a German Web portal which provides an email service with address books and calendars.
Two separate DAVx⁵ accounts are required (one for CalDAV, another for CardDAV) because there are no SRV records.
- either use "Login with email address", or
- base URL:
https://caldav.gmx.net(or instead of .net please use your respective .tld)
For CardDAV, use this base URL:
https://carddav.gmx.net (or instead of .net please use your respective .tld)
User name: your GMX email address
Password: your GMX password; if you use 2-factor auth you will have to generate an app-specific password and use this instead of your normal 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.