I'm with Jack and John on this part. If you return the check but they "lose" it, you have no proof at all that you sent it. Registered mail does not prove WHAT was in the mail. Sure, they have no proof that you cashed it, but since they have all the power, they expect YOU to prove that you did not, On the other hand, if you cash it and put it into a savings account,, AND you keep copies of all the correspondence where you try to clear it up, I've never had any problem getting them to abate any penalties. If you do hold it until you are certain that it is not a proper refund, then return the check at a local IRS office, in person, and get written confirmation from them that you did, signed and dated, Paranoid? Maybe, but based on real-life experience.