Interesting concept. Certainly makes sending hand written notes easy. Is the UI (changing an email to a note, rather than using an app or website) enough to differentiate this service.
This summer I did several hundred cold calls and several thousand cold emails.
On the sales front, I found emailing a much better use of time. While sales were marginally easier on the phone, getting the right people on the phone beyond difficult. And even when people agreed to chat - they often bailed last minute or never called me.
With emails I found two techniques -
For sales emails, I found the best response rates when I was the weirdest. For example, in a moment of exasperation after emailing the same guy 4 times (and getting no response), I wrote "Maybe you're like the wizard of oz, everyone lists you as a media contact, but you don't exist." I heard back within 30 minutes.
For intro cold emails, I found people incredibly receptive. The more specific the emails (specific to the person), and the more concise the email, the more warmly I was received. Perhaps common sense.