A query letter--the one you write to entice a publisher or agent or producer to request to see your manuscript or script, or to commission you to write an article--usually is your first point of contact. If it turns off the recipient, you're done.
Recently an agent pointed out one of the big mistakes people make in query letters. She said: "Do not ever mention any kind of rejection in your query letter. Ever. This is a hard and fast rule."
So even if you received a rejection from a publisher that said, "We really like the writing, but this is too close to something we are already publishing," don't mention it. Don't write, "This manuscript was nearly accepted by Publisher XYZ, but turned down only because they had something similar in the works."
Why? Because sometimes publishers lie. (I'll pause here to allow time for your gasps of astonishment.) They do it to spare your feelings, but even more to spare them from the wrath of rejected writers.
If they wrote, "We are rejecting your manuscript because the protagonist is unrealistic," there are certain writers who would write back, "Listen here, that protagonist is based on my Uncle Dexter, how dare you say he's unrealistic! I am enclosing six pictures and a letter from Uncle Dexter himself, attesting to the fact that he's real."
Of course, sometimes it IS true that they're rejecting the work only because it's too close to something they're already doing, or because they don't handle that particular genre, etc. but even so, it's not a good idea to mention prior rejections. The exception is that if an agent offers to represent a project, it's only fair to let him or her know where it has already been seen.