I'd be incredibly sceptical of the service.
The service kind of assumes that the emails are very confidential in nature that can't be sent using regular email systems. And by that reasoning, there is serious lack of trust oozing from the website. It's using buzzwords but doesn't go into detail much. Who's accountable in case of a breach of any kind? Encryption, sure, but the keys must all be stored somewhere... And that alone is the problem.
It reminds me of Nulled themes and plugins provided here on BHW.
So much for feedback, sort of a random guy's biased perspective and feelings towards it.
For me right now, it's comparable to trusting some totally, absolutely random little menacingly laughing kid yelling from super safe distance: It's safe! They won't bite!" whilst having to cross a visibly and purposefully shark infested body of water.
So to bring out a bit more trust, I'd like to see in detail a sort of comparison between a regular email vs your email system. In what way exactly can a hacker read my emails when I send it via a regular email system and how they absolutely can't do the same thing with your service. The more of these kind of examples you can come up with, the more trust it builds. Ultimately every inch of the possibilities would need to be covered like that so as to instill utmost trust.
I'd aim for squashing all the potential doubts of the potential clients, essentially.