Unsent Letters are letters, and as such, should be written in letter format, as though personally written to another person. It is not necessary to start the letter with "Dear so and so," as long as the letter is clearly written from one person to another. It is acceptable to write the letter to more than one person; for example: your children, your parents, etc. but we would like to do most letters to one specific person. We prefer the letter be 'true', but you are welcome to change names or locations as necessary. If the story is compelling fiction, as long as it meets the guidelines, we're okay with that too, as long as it feels real to the reader. It's the emotional connection the reader can find with you, the writer, that we're looking for.

Unsent Letters accepted for blog posting, $10-25 per letter, payable within 30 days of publication. A live link in your byline on the blog when your story publishes will also be included. Unsent Letters accepted for the published collection (paperback anthology) receive $50-250 per letter, depending on length, appeal, current budget, category needs, editor's needs, and how much formatting and editing will be required to make the Unsent Letter publishable.  

from: www.hopeclark.blogspot.com