The Pony Express delivered messages, sort of like email, but transmitted with ponies! Or Twitter on four legs! When a post was delivered, there would be “postmark” put on the mail, showing it had gotten to where it was intended to go.
Design a postmark stamp for the Pony Express of the Modern Age.
See some examples for inspiration: https://commons.wikimedia.org/wiki/File:Postmarks_Pony_Express.jpg