Would the correct form for a "signoff" be "Yours sincerely", rather than "Yours Sincerely"?
I have always written capitals for the salutation, e.g. Dear Sir, but never capitalized the second word in the closing.
Mike
Only the first letter of the first word is capitalized in the complimentary closing:
Sincerely yours,
Very truly yours,
Yours truly,
etc.