need to know more details ie it is .com or .co.uk. The thing is if the term you register as a domain name is generic or something that you have a right to use. Then go on. use it. ignore that letter/email but keep everything as evidence in case you r filed with a complaint. (if necessary, print it out) The trend nowadays is to deny a complaint except it is really obvious that you r squatter. lol
Perhaps, you can talk with this guy at motorz.com, he was filed to take back the domain name by motorola but I think he has a gut to fight for what he has and he won. Miracle, isn't it, with Motorola.
Anyway, if there r more to ask n u feel it is private, your welcome to pm me.