Recently I heard about someone who'd received a rejection from a literary agent. The agent had given the author advice on how to get her work up to scratch. This advice--I don't know whether it was warranted or not--didn't go down well. In fact, the author sent a blistering, bridge-burning reply.
Some rejections kill. The good R's are those that tell you where you've gone wrong. You're a professional. Don't reach for the poison pen (or keyboard) to tell the agent or editor they're in the wrong business if they can't see how fabulous your book is. You just might give yourself another handicap in getting out of the slush pile. It's a tough biz. Kvetch to your friends in private. Fantasize all you want about "getting back" at The Agent Who Hurt Your Feelings, but don't act on it. If you must correspond with the agent at all after a rejection, a simple thank you will do. Then move on.
People in publishing talk. You don't want to be remembered for all the wrong reasons. Argue a point after a rejection and chances are your next query to that agent or agency will be trashed. Is getting the final word really worth the trouble?
To prove there are hundreds of other agents to query, check out this terrific new resource called Lit Match. It's a huge database of US, UK, Canadian and Australian agents and you can also register to keep track of your submissions.