@SangheiliSean:

Last I checked, a moving services is not the post office.

Mailmare? More like failmare. :^P

Seriously though, the "Derpy = Mailmare" thing is about as far from canon as you can possibly get, and the writers clearly don't like the idea. They've had plenty of opportunities to have Derpy be a letter carrier, and every time they've always used a different character, which tells me they have no intention of ever making Derpy be a mail carrier.