That's some intense mail, and I also like how you appear to have drawn every one individually as opposed to copying and adjusting. Commendable. As for Derpy, I like the expression and character but the head is a bit large less you intend to convey her age that way. The highlights in her eyes could use some slight adjustment as well. A little overlapping into the iris or otherwise spreading them out a bit would probably help. The eyelashes are also a little fake. They're too bold and don't come to a point as eyelashes normally would. They're also angled too sharply upward and are strangely spaced. I like the lack of outlines unless necessary in this image as well. She doesn't pop out much from the background but she doesn't in fact need to given the composition.
I hope this was helpful.
P.S. I don't like this rating system for this subject matter. I prefer you ignore the rating as I won't be genuinely using it.
And from this day on... all she saw when flying and delivering the mail.... was muffins. That and occasional ...bubbles? O-o () This is like some standard poster describing pretty much everything about our sweet muffin munching mailmare I can't say for certain but it LOOKS like every muffin and even every mailcard is unique! Nice!