When you visit and use your GMX mailbox, the resulting data is transferred via the internet. This data contains information such as your e-mail and credentials. To ensure that this sensitive data cannot be read while it is being sent, it is transferred encrypted.
This step is performed by the "SSL" (Secure Sockets Layer) transfer protocol or its successor "TLS" (Transport Layer Security). Your browser (for example, Firefox, Chrome or Internet Explorer) agrees a shared session key with GMX, which is used to secure the data in both directions. The data can be made readable again only by using this key.
This process takes place automatically. It does not involve costs or extra efforts on your part.