As people have said, there is always a way from someone to intercept a message. An organization can, at the ISP level, intercept all images that you access in your browser and then perform OCR on the images you receive to extract the message. So there is no foolproof solution here. However, this is simply a service that allows you to send "read only once" emails, with no SMTP records of the message on any servers. It has a certain level of protection that I feel some people will find useful, in the same way that some people really value off-the-record gchat.