They might be using scare tactics hoping you'll hand over the domain name without question. But I wonder if they have a case. My suggestion would be to either A) ignore and see if they go away, and/or B) have a domain name lawyer send a reply showing you won't roll over. That's usually enough for parties like these to back down. If you need a lawyer recommendation send me DM me on Twitter (@marckohlbrugge). More than…
Marc, do you have an email address?