Thank you for your feedback.

We are looking into the described behavior and are trying to reproduce it.
Could you please send us an e-mail to mozdev@1und1.de? We would be grateful to receive more details as follows:

1. MailCheck version
2. Firefox version
3. Are you using MailCheck for international email accounts (gmx.com, gmx.co.uk, gmx.fr or gmx.es)? Or do you have an German GMX-account?
4. Is it possible to login directly on the GMX portal? Do you get the same message there?
5. Do you use other extensions, i.e. NoScript, that block website content? In other cases this caused similar behavior.

Providing screenshots or videos via mail could be helpful to solve the problem quickly.

Best regards,
Your MailCheck-Team