So seek the advice of a good patent attorney who has seen this sort of thing before.
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.
A legal claim demands a legal response, there are no shortcuts here. Your lawyer will advise you further.
so dont drag your feet, approach a lawyer[s] and find one prepared to make a good effort.
it sounds like you are in a position that begs some legal assistance as part of regular operations, you should have one in your "rolodex"