In law, the copyright of any unpublished letter belongs to the writer or his/her heirs for ever. Once it is published, copyright in the letter ceases to exist seventy years after the death of the writer. This, even if it is for business purposes, remains an unpublished letter, and only the writer, (the author) , has the right to publish it. There is no need to say so on the letter, as the law exists as I have described it.
If the person to whom the letter has been written is the editor of ( say) a newspaper or journal, the writer is making it clear that this is not a contribution for the publication.