The whole "mail carrier" thing is just "fanon." It's never been established in the show that Derpy/Ditzy is a mail carrier, or delivers mail. (In fact they showed a different pony does that in the episode "Putting Your Hoof" down.)I suppose if fans like to think of her as one, that's ok to, considering Derpy being a background/gag character alot of her character is left to the fans' imagination.