When Derpy Hooves has a baking session with Pinkie Pie, she gets Rainbow Dash to deliver mail for her.
While Rainbow Dash has trouble delivering mail, things aren't going well with Derpy and Pinkie Pie either.
(Rainbow Dash doesn't have her cutie mark in the picture because it is on the other side of her body) (Okay, now she has her cutie mark. I just got lazy, and forgot about that. I tried fixing it, but my internet was screwed up for a week.)