Mailvelope is a german security-partner of GMX, which allows you to manage your encryption data through a browser extension.
During setup you are asked to download and install the Mailvelope browser extension. Mailvelope is a german security-partner of GMX and specialised on email encryption. GMX guarantees that the Mailvelope browser extension is a trustworthy software.
As soon as you completed the setup and are logged in to your GMX Account, you will have the option to manage your keys in the Mailvelope browser extension. This will be no longer the case, when you manually delete the respective key in Mailvelope.