Poor man's copyright is a cheap, but a much less effective way of protecting your work.
It is achieved by sending the artistic work to yourself and instead of opening the envelope, you leave it closed.
The postal stamp would then serve as a proof that you're the owner of the work.
However, poor man's copyright can easily be forged. You could send the envelope to yourself without properly sealing it, then at some point in the future enclose any work you like.
You could also change the date stamp on the envelope.
It is for these reasons that poor man's copyright is not a very effective method of protecting your work against infringement, In a court of law it is very unlikely that it would prove your copyright ownership.
It is much better to go for the far more secure proof that is provided by an independent, third-party copyright registration service.
Copyright House provides an easily affordable, independent third-party proof of copyright registration.