Among many artists and writers, there is a common rumor that you can obtain copyright protection for your work by placing it in an envelope and mailing it to yourself. The idea is that you now have official proof that you were the first person to create this work, based on the postmark on the envelope.
This rumor makes some intuitive sense. However, it is entirely without merit, and will provide no additional protection for your work.
Your creative work is protected by US copyright law the instant it is created and fixed in any tangible medium (e.g. a manuscript, photograph, painting, sound recording, etc.). This is not to say that registration of your work with the U.S. Copyright Office is not useful. Registration serves as almost irrefutable proof that you own a valid copyright. It is also required before owners of works that originated in the U.S. can sue for infringement, and allows all other owners to collect significantly more damages than they would if they had not registered.
Poor man’s copyright cannot be used as a method to prove that you were the first person to create a work. In fact, no American court has found the poor man’s copyright method to be of any probative value. This makes sense because it would be easy to fake by placing new materials in a previously-mailed envelope (the U.S. Postal Service does not require envelopes to be sealed).
Although copyright protection is automatic, you may still wish to register your work with the U.S. Copyright Office in order to ensure protection for your work. An intellectual property attorney can assist you with this process. You should also seek legal guidance if you believe that you are a victim of copyright infringement.