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