According to U.S. Postal Service regulations, anyone who uses a private mailbox service must complete USPS form 1583, which you can find, along with instructions on how to fill it out, at:

http://www.mailboxforwarding.com/form1583.php

After signing up for an account, you will receive a welcome email containing a link to this form. The U.S. Postal Service uses form 1583 to ensure that private mailboxes are not being used to commit fraud, and we cannot scan or forward your mail until we have received this form.