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 to edit it you have to defer to next pickup".
I would think a "mindful correspondence" system would have an enforced gap between composition and delivery to avoid "oops I clicked send at 6:29 PM but it's wrong and I need to change it".
I mean, this is based on my experience in email and twitter, which you've rightly identified as the opposite of "mindful" correspondence (where "submit" and "deliver" are the same action), and personal experience with business apps that make heavy use of fixed daily jobs (where "submit" and "deliver" are the same action at one very specific deadline that everybody is rushing to hit).