Dear Princess Celestia,
I am writing to you today about an urgent matter: I THINK DERPY HAS CAPTURED SPECIAL DELIVERY, OUR NEW MAILPONY CARRIER! PLEASE SEND THE ROYAL GUARDS TO INVESTIGATE THIS! WHO KNOWS WHAT SHE'LL DO TO HIM!
Princess Twilight Sparkle.
Looks like Derpy got tired of waiting for the Postal Union's decision on the matter, and took matters into her own hooves.
I can imagine the letter Twilight was thinking about would say:
"Dear Princess Celestia,
Derpy kidnapped Special Delivery and has become the new mail pony. Send in the Royal Guards ASAP!
Your faithful student,
Only way Derpy will EVER get another chance is if they change her name and underp her eyes.
(BUT THEN SHE WOULDN"T BE THE DERPY WE KNOW AND LOVE)