Two-factor authentication protects your GMX mailbox. After successfully setting up this security feature, not only will you be prompted to enter your password when you log in, but also an additional confirmation code that you will generate each time using your smartphone. This provides you with additional security: Even if unauthorized parties obtained your password, they could not access your mailbox. Your data remains protected.
Additional security requires additional effort as well. An authentication app is required to set up two-factor authentication. It generates a unique, temporarily valid security code each time you log in. If you do not yet have an authentication app installed on your smartphone, please visit Selection of apps for 2-factor authentication for an overview of available free and paid apps plus links to additional information.