Despite her usual Walking Disaster Area tendencies, Derpy's genuine aptitude for mail delivery (at least when not flying) is demonstrated when she completes her rounds on her first day in Ponyville well ahead of schedule, even after the hours she lost waiting for the letters to dry after Twilight scattered them in the rain - helped by the fact that she was hours ahead of schedule even before that accident. It's easy to see why Post Haste was so eager to hire her.
When Rainbow Dash reacts with open scorn at the idea of Twilight being friends with a Walking Disaster Area like Derpy, Twilight leaps to her defence:
Twilight: I seem to recall plenty of times where youíve been an accident waiting to happen, Rainbow Dash. If my memory is correct, the first day we met you plowed me into a mud puddle, drenched me with a raincloud, turned my mane and tail into a frizzy mess, and laughed at me the entire time. Donít even get me started on how many times youíve destroyed my library. And, regardless of that, Iím still your friend. I hope you can think about that for a bit and next time at least be a little more civil.
Fluttershy chews out the jerkass Cloudsdale mailponies for picking on Derpy.
Fluttershy: You think that you are so much better than her, and that you have the right to sit here and laugh at her? That you can track her down simply to drag her through the mud, so you can feel better about your pitiful, shallow selves? That you can use her like a punching bag to fill the void in that wretched, meaningless existence you call a life, so you can face tomorrow feeling like youíre more than the worthless scum you are? I swear to Celestia, if I ever catch you Ė or any of the miserable vermin from your ratpack Ė insulting her again, I will personally ensure that you will not live long enough to regret it. Do. I. Make. Myself. CLEAR?!
The stallions whimper and nod.
Fluttershy: Then get out of my yard and donít let me ever see your useless, vile, sorry excuse for carcasses ever again!