How can they stop you taking a screen-grab of the e-mail and saving it? Or just grabbing a photo using another device?
And what company is going to be able to legitimately use self-deleting e-mail? There are all kinds of law regarding audit trails and data retention.
Not sure how either of these systems are going to appeal to anyone other than the likes of Uber and other scumbags.
Or rather, the idea may appeal but the reality will prove to be somewhat different to the blurb in the sales brochure.