Moreover, I very much prefer my e-mail textual - some of my e-mail devices may be severely constrained in bandwidth and screen size; text compresses, transmits and scales way better (also: insert standard accessibility rant here).
Also, what of nomadic users? "Oh, your smartphone already accessed the one copy [for added fun, try "and didn't save it"]? No way to read the e-mail anywhere else, tough luck."
You have addressed the above as caveats - however, there one more thing that bothers me, immensely - the immediate, silent and complete retraction capability: "I never said that" is bad enough, "I never sent you an e-mail like that" would be worse. For dealing with certain people, I like to have a local copy of what was written, just in case they change their mind later. Even if I kept local copies of the screenshots, I like my evidence searchable, too - eyeballing a bunch of images to find a specific e-mail is distinctly suboptimal.
On the other hand, if you are facing the one exact problem of people mindlessly forwarding your e-mail, verbatim, this might be a useful mitigation technique. It's a nice project, but not useful for me - it would solve problems I don't have, while saddling me with other problems I don't want to have.
As for "no real website design" - I actually like the clean and minimal design :)