Assuming that you have already verified that the financial institution actually exists and that you once did business with them at some level, common sense would dictate that you respond by asking for documentary evidence of the debt. Make sense?
Does the letter say when the debt was incurred? How much are they asking for? MO has a long statute of limitation for a lawsuit based on a written contract (10 years). Is the account still active? Do they have any of your money that they can use as a "set off" if you don't pay?
What you do or don't do depends on a variety of factors that you haven't yet shared.

