This is a prequel to undeliverable. Why did Derpy go missing? Where did she go on vacation? Only one pony has the answer, the same pony who is dictating the letters. She is Twilight Sparkle, the Princess of Friendship.
The contest now has its first entry, and the early bird has earned his worm!
Redeliverable
By Emotional Flight
A collection of letters from Ponyville's Old Mailmare.