Hollywood Experts have said to avoid doing certain things in query letters. All of these things will probably result in them deleting your query. 

Avoid using humor. Sometimes trying to be funny only backfires and they will usually immediately stop reading and delete. If you’re a comedy writer, it’s most important that the logline be funny.

Don't kiss ass. While it's nice to show respect, kowtowing to an agent or exec is unprofessional.

Do not include scenes or quotes from your screenplay. Enough said, just don't do it.

Avoid insignificant praise and other unnecessary or stupid information. Never include reader's comments and such things. Also, avoid describing your script with hyperbole. Sentences like, “This is an Oscar winner” or other non-semse.  No one cares if you’re a retired whatever.

Proofread the letter.  Query letters are often littered with misspelled words. This also includes grammar and syntax errors. 

Letters should be sent to a specific person. Be sure their name is spelled correctly. Refer to the “Hollywood Creative Directory,” the Internet, IMDBPro or call for the correct spelling. In general, calling ahead is a good idea. Double check to make sure the executive is still employed with that company. Call ask for the person or go to their website and male sure still listed. Do your research it shows you atleast gave an effort

Keep it short a few sentences. Remember you are pitching your story.

In the title use "RE:" do not put Query, unless you know for a fact this person or company is open to email queries.