You go on https://www.guantanamail.com : we use a SSL certificate generated by a swiss company and all our data are stored in Switzerland. Every communication between you and the server are therefore encrypted, even if we have added further protection to your data. By the way, every tool we use are stored on Guantanamail servers : there is no third-party request.
You write your message, we encrypt it client-side with a 60 character key randomly generated. The encryption system is AES-256 version from the Open-Source library http://code.google.com/p/crypto-js.
We split your key in two part of 30 characters and we generate an 60-caracters ID for you message. We send this ID, the first part of the key and the encrypted message to our Swiss-based server using a secured SSL connexion. No other data is stored about you.
We give you the second part of the key and the 60-caracters ID merged in a single access code. You can send this code using Guantanamail mailing system or use your own secured channel to your contact.
The receiver requests the encrypted message using the ID you sent him : he decrypt it client-side and the message is immediately deleted from our servers. He has 180 seconds to read it before web-page refresh. At the end of the process, there will be no trace of your exchange.