Please update your GMX.com MailCheck to the latest release 5.0.2. This release from the 25th of May, 2020 also supports gmx.us among other additional domains.
If you still are not able to login with your email account, please contact mozdev@1und1.de with details on versions of MailCheck and Firefox and how you proceed.
If it works, we would appreciate the correction of your rating here.
