The numbers

No made-up speeds. Each messenger moves at a pace pulled from a real record, averaged over a whole day, nights included. Distance is measured straight across the globe, city to city. Real ships and caravans took the long way round, so if anything, your letter is getting a head start.

Sources

Also

Letters are encrypted in the sender's browser before they leave. The key sits after the # in the link, and browsers never send that part to the server. So the server holds a locked box and a timer. It hands the box over when the timer runs out, and only the link can open it.

No email gets sent to anyone. You share the link yourself.