Copyright Tonya M. Evans. All rights reserved.
Excerpt from Literary Law Guide for Authors
The short answer? It CAN’T. Surprised?
The concept known as the poor man’s copyright says basically that you should mail yourself a copy of your work and not open it in order to establish the date that your work existed in case you ever need to defend your work or challenge a work you believe infringes on your rights.
This, unfortunately, is a myth that continues to be perpetuated even by established writers and it should no longer exist in the writing world!
The Poor Man’s Copyright does not offer any additional protection beyond that which already exists once your idea is fixed and thus your work is created. Additionally it does not constitute a registration of your copyright.
Registration is generally straightforward and inexpensive (as low as $35, as of this posting) – a small price to pay to preserve valuable rights. Consider it an insurance policy. For more information about copyright, visit Copyright.gov.