Info about cashing fake checks:
https://www.fdic.gov/consumers/consumer/news/cnwin0607/scams.html
Mail Fraud Form: http://ehome.uspis.gov/fcsexternal/default.aspx
As was pointed out, take the check to the post office and have them forward it to the postal inspectors. Do not deposit or try to cash that check. At the very least you will incur bank fees. At the very worst, there is a possibility that you will be criminally charged.