1) Would you send a substantial check to someone you did not know in hopes they would do something for you? 2) If it looks like a duck, walks like a duck and quacks like a duck, why would you assume your white feathered friend was not a duck? Report it to the local police and if you want to be bothered, to the MSPA.
Of course the sender is hounding you! He wants to wear you down to report the card numbers and pins back to him so he can empty them before his check bounces.