The appropriate step, as presented by the experts, is to notify your ISP or local FBI office. You can use the link below, if you have a website address which the email came from, (not an email provider), otherwise, email is not reported to an agency outright because the email information is usually spoofed. You need to go to your ISP because they can track the sender more accurately.
If you receive such material in the U.S. Mail, you need to go to your post office and give the material to the supervisory postal agent. Reporting child porn found on a website