You keep them with your records. You do not mail them to the IRS.
As stated on the back of the W-2 -
Keep Copy C of Form W-2 for at least 3 years after the due date for filing your income tax return. However, to help protect your social security benefits, keep Copy C until you begin receiving social security benefits, just in case there is a question about your work record and/or earnings in a particular year.
