From an agent:
Most of us agents and editors are real schoolmarms when it comes to grammar, spelling, and punctuation. And usually--to be blunt--we know far more about these matters than the average aspiring author. IT IS ESSENTIAL that an author have the query letter, proposal, and sample chapters proofread thoroughly, preferably by a literary professional, before they send them in to us. Even that friend of yours who is the local high school English teacher, sad to say, is usually not good enough to accomplish this task properly. I'm not looking for writers who are going to require me to spend time correcting their manuscripts; that is not my job.
And if I sound pissed off about this, it's because after years of such submissions, I AM!