Apparently gmx’s servers have an outdated architecture, which has something like DDoS protection for IMAP requests
GMX servers work fine with eM Client
Unauthorised for GMX accounts with IMAP is usually to do with the Two Factor authentication that either “needs to be disabled” in your GMX account or “a new 2FA app password generated” in your GMX account for eM Client as per this thread.
If disabling 2FA in your GMX account doesn’t work or generating a new app password in your GMX account also doesn’t work, then update what IMAP account server settings you have in eM Client so we can check if there is any obvious errors.
Also update what version of eM Client you have and if you have Windows or Mac & OS version.