It is established etiquette by the Joint Chiefs of Staff, United States Department of Defense, the U.S. Army, the U.S. Navy, and the U.S. Air Force that the closing “Respectfully yours” be reserved for the President of the United States (according to U.S. Army standards, this also extends to the first lady and the President_elect).
Strike the Right Tone: Make the purpose of your letter clear through simple and targeted language, keeping the opening paragraph brief. You can start with, “I am writing in reference to…” and from there, communicate only what you need to say. The subsequent paragraphs should include information that gives your reader a full understanding of your objective(s) but avoid meandering sentences and needlessly long words. Again, keep it concise to sustain their attention.