I really think you can send a C&D order for anything. Technically, I could send you a cease and desist order demanding that you immediately cease using the word "the" on your web site. These letters are generally requests, although if there is a legal basis for the request they can be the first step in a lawsuit, and a necessary step in many courts if damages are being sought. That said, C&D orders that don't have a legal basis could be used just for intimidation, to prevent you from competing with the sender, which I think is the case in your situation.

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The best way to learn anything, is to question everything.