I get attorney's letters all the time. I am in business for myself, and every now again, an attorney will threaten to sue me through regular mail.
Unless you are properly served a summons, I would not worry about it, but do write them a letter, certified receipt requested, that the debt is not valid, and for them to stop contacting you unless it is
1) to let you know that they are dropping everything against you.
2) to let you know that they are pursuing additional action against you.
Case closed..
and if they somehow are able toput anything on your credit report, again, write them a letter and sue them.