Take the check and the envelope it came in to the post office and have them forward it to the postal inspectors.  This is a common scam.

Job Scam:
How can you tell if this job offer is a scam?
1.  There is no personal interview, or sometimes no interview at all.
2.  It is a work at home position many times because the office in your area is not built, rented, or established yet.
3.  You need a bank account to receive and deposit checks, or you need to receive packages and resend them.
4.  You need to deposit the check, and send the money through some way such as western union or Greendot money card.
5.  The pay and benefits are way beyond anything you would normally earn.
6.  All communication is by e mail, text, or instant message.