That's a charming idea. I could see how email bridging might be a way to achieve network effect, but then email bridging might just turn it into another email client. I'm surprised you took the approach that pickup = delivery. I would assume you'd have a lag between them, with only cancellation allowed in that gap. Basically "you have N minutes to reconsider this thing you wrote, if you've changed your mind and want…
I'm not at all involved with this, but just based on reading the website and reading your comment I think you might have a slight misconception of how this works. There is no send button. You write drafts, and then at the pickup/deliver time, Pony picks up all the drafts from your outbox, and delivers any mail for you at the same time. Therefore, you can change your mind about what you've written at any point before…
So if you compose and submit a draft at 10 seconds before pickup/delivery time, you have no time to reconsider. That's what I'm getting at. I would assume a "no drafts submitted before X minutes before pickuup" rule to mandate a minimum reconsideration time.