Check fraud is one of the most common types of fraud that occurs on a day-to-day basis. If you receive a check in the mail from someone you don't know, it is most likely fraudulent.
Scammers are known to send fake checks to unsuspecting victims, whether it be for an item purchased online or for services not yet rendered, usually for a larger sum than originally indicated. They will instruct you to deposit the check into an ATM or by using mobile deposit then ask you to withdraw the funds immediately and send the money back to them, often telling you to keep a small portion for your troubles. By the time the bank finds out the check is fraudulent, the money is gone and you may be held liable.
Take a look at this infographic provided by the Federal Trade Commission to learn more about check fraud and how to avoid it.