Comment by DamonHD

Someone I know who is a patent lawyer suggested that if I found myself in this situation asking for more details of the claim, calmly, to determine if it has substance. But depending on the counterparty's motivation that may or may not help.

Make sure that you get enough legal advice to avoid digging yourself a deeper hole: you may be wanting to mark such correspondance "WITHOUT PREJUDICE" or similar for example. IANAL and that is not legal advice.