Here's what turned up at PubLaw:
Allegations involving copyright infringement frequently occur when the author of an unauthorized biography makes use of the subject's published or unpublished letters and papers or possibly from oral conversations the author may have had with the subject.
In Salinger v. Random House, Inc., the author's use of extensive quotations from unpublished letters written by J.D. Salinger, the subject of the biography, without Salinger's permission was deemed to be copyright infringement. Under copyright law the writer of unpublished letters has the right to control the first publication of those letters.
Criticism of the Salinger decision as well as other copyright infringement decisions based upon what was believed to be excessive protection for unpublished works, resulted in Congress amending the Copyright Act. U.S. copyright law now provides that if a work is unpublished and it is used without permission of its author then the fact that the work was unpublished by itself would not be sufficient to constitute copyright infringement.
Subsequently, in Wright v. Warner Books it was held that when a work is one of "criticism, scholarship or research," that quotations from unpublished personal letters and journals might constitute fair use.
In Estate of Hemingway v. Random House, Inc., which concerned the publication of portions of conversations between the author and Ernest Hemingway, the court ruled that the author of Papa Hemingway did not infringe the common law copyright of these conversations by including them in a book on Hemingway's life. The court stated that even if the author used verbatim some of Hemingway's words that such use was minimal and qualified as fair use of the material.
Emphasis is mine to highlight the parts that seem to apply to what we're talking about. I think I was confused when I originally said "letters belong to the recipient"; confused between the concept of ownership of the actual physical letter, and ownership of the content. It does seem clear that the original commenter (and Writers Digest!) is correct in saying copyright attaches to the creator of a work even if it's a personal letter.
But copyright in and of itself is not at issue. What started the exchange was the idea that I'd have to "get permission to use the letters" if I compiled a book on bad queries. (Don't worry Snarklings, Miss Snark isn't intending any such thing). Even if I printed, verbatim, every query letter I received for a year (A Year In the Life of Miss Snark!), you'd be hard pressed to sue for damages (if such a loathsome thought occurred to you) because you can't sue for damages unless you register your work. Invasion of privacy maybe, but not copyright.
Copyright violation is a civil action, not a criminal one. The State of New York will not enforce the civil code against copyright infringement on your behalf, but it will enforce the criminal code of ..for example..no murdering literary agents, on my behalf.
But, by sending me an email, you grant implicit consent that it can be published on this blog. So watch out; no threats to stew Killer Yapp or I'll sic PETA on ya!