Stolen checks have become a big problem. A new revenue stream for organized crime. You should immediately follow up with your bank, and maybe stop payment. A few weeks ago, my buddy sent two checks totalling a few hundred dollars. Mailed inside the post office. They were stolen, altered, and cashed for several thousand. He's working with Postal Inspectors and Navy Federal Credit Union. He thinks he'll probably get refunded, but it's been a huge hassle.