When you receive a mail document, it can be encrypted. When you try to access encrypted data, you will raise an exception. To prevent this, you might send the isEncrypted message to a document and, in case you receive a true object, you might try to decrypt the document using the decrypt method. Decrypt will try to decode the encrypted data using the decryption key included in your ID file. If you do not have the appropriate keys, the message send will raise an exception.