Good question. Security and confidentiality is paramount to this service.
To give you an idea how this works, when you upload a letter, the system stores it as an encrypted file in the cloud. The system then uses a third-party faxing service to send the fax. Once the fax is transmitted it is deleted from the third-party service and also from the cloud storage. This is the default behaviour of the system. However if you like to keep a history of your letter, simply change it in the settings and it will be kept after they are transmitted.