You can decrypt encrypted e-mail by opening it and entering your passphrase.
You will find encrypted e-mail in your inbox among "normal" e-mail. Note the lock icon.
Click the encrypted e-mail.
Enter your passphrase
The mailvelope addon will encrypt your
e-mail. This process takes place locally on your device, no unencrypted data will be
sent to or from GMX servers or other remote machines.