The Copyright Office refers to Poor Man's Copyright as:
The practice of sending a copy of your own work to yourself is sometimes called a “poor man’s copyright.” There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration.
With copyright law, you own copyright as soon as you put your original work into a fixed format. You can take the step to register copyright with the Copyright Office if you want extra legal protections.
