Never enter your credentials, passwords, credit card or contact details in an unencrypted email. Do not pass this data on to third parties in any other way.
Be careful when entering your email address and password after you have clicked a link in an email. Be careful even when the email or web page appears to be from GMX. Always verify the web address and sender of the email.
If you do not know the sender of the email, do not open any file attachments in this email. Make sure that the file attachments are secure even when you know the senders.