Here's the text of the original post, and the comments are definitely worth reading as well:
It might be silly to even say this, but I highly advise not insulting agents or editors when responding to rejection letters. Even if they're mean or belittle you, even if you feel like a great injustice has occurred, please just take a breath and move on.
However, if they've treated you in a way you think might not represent the best interests of their publication or company, do figure out who to email that might supervise the agent or editor. Let that person know you've been mistreated.
This happens way too often, and it doesn't make sense to me because my rejection letters usually read as follows:
Thanks again for submitting to [publisher]. Your manuscript had [specific thing that worked well] and [maybe another thing that worked], but because of [reason for rejection] I've decided to pass on it for me.
In today's rejection that prompted this post, I'd given a simple reason: the prose wasn't where it needed to be, that another round or two of revisions were necessary before I could get behind it. The author responded by trying to make me feel bad, shaming me for saying their writing wasn't good enough to be published.
And that sucks for them because such an attitude is going to get you absolutely nowhere if your goal is to find traditional publication. Even if I'd been mean or cruel, it would have been a bad move on the author's part to send a defensive response (or in this case, an attack, since the author seemed less concerned with the possibility of the truth of my assessment and more so with letting me know she didn't approve of how I handled things).
Normally I just delete these emails and move on. But today I recalled how many times I've been in a workshop or read about an author sweeping their Amazon review page or whatever else with the intention of defending their work. You don't need to do this. Your work should stand on its own, to use the cliche (which you shouldn't do in your fiction :P). If an agent doesn't like it, for whatever reason, that means they're not your agent. If they were mean to you, that means they've even less your agent. Sucks for them. Move on and find your agent.
I want to help authors. That's what editors and agents do. We help authors get published. It's really hard to do this job when authors have shitty attitudes. Agents and editors can have shitty attitudes too, and that's bad for them and the publishers and clients they work with, but a quick and easy way to drastically increase your ability to be published is to treat other people in the industry how you'd want to be treated. Don't lash out at an editor because of a rejection; stop and listen to the reasons for the rejection. This author could have instead asked for clarification, which I would have happily provided. Instead they got their email deleted and have less of a chance to get their book published.
TL:DR: be nice, yo.