The Equestrian Postal Service used to be MUCH more dangerous.

"Now that we've invented paper," Princess Celestia tried to explain to her sister Luna, "we can send a message to another pony without the necessity to write the message on a THIRD pony, let alone ordering that third pony, upon pain of death, to seek out the recipient and display their flank."

_{(cover image source: I cropped and edited part of an official MLP comic to make a better cover image for this story.)}