letterhead

I'm not sure what you mean about "validity".  A cease and desist letter is not a court order; it is simply a letter, in this case written by an attorney (apparently) representing a party who claims to be aggrieved.  You are under no obligation to obey a letter, although to the extent that the activity complained about is later deemed by a court to be wrongful, and you continue it after being put "on notice" by the letter, it could affect a Court's view of your conduct and the amount of any award against you.  I know of no requrement, ethical or otherwise, that requires a lawyer to write letters on letterhead, although I agree with you that it is unusual for such a  letter not to be on letterhead, and you may be right to question whether the author is actually an attorney.