I am not a lawyer, this isn't legal advice. Definitely don't respond yourself, get a qualified lawyer in with track record in startups/domains to write back (an

I am not a lawyer, this isn't legal advice. Definitely don't respond yourself, get a qualified lawyer in with track record in startups/domains to write back (and advise). The letterhead goes a long way here in showing you're not trivial to push around. If facts as presented here are truthful, there is a reasonable chance of never hearing from them again.