Casn you send a debt validation letter? Sure. But, they can ignore it. Why, because the collection agency sent you a letter to your last known address which is all they have to do. Almost all credit applications have a clause that requires you to tell inform them when you move - you obviously didn't, so as far as they know, your home address was your legal address, so that's where the letter was sent - case closed, requirements met. If you go to court (of they sue) you can ask for proof that they own the debt (collection agencies have gotten smarter about keeping proper paperwork so this usually doesn't work anymore but it can't hurt to ask).
In the meantime, pull your credit report - you should see two defaults - one from the original creditor and one from the collection agency. Take it from there.