Trying to copyright register your song through the mail is known as "poor man's copyright." The steps to this type of process include: creating an original song; putting it in a fixed form (i.e. music notes on paper, sound recordings on a CD); placing the finished song in an envelope; and sending it to yourself using registered mail.
Unfortunately, and contrary to popular belief, this method simply does not work in the eyes of the court. While in theory, "poor man's copyright" seems like a great option to protect copyright, in practice, it's not.
The inspiration of sending the song to yourself through registered mail is that you will acquire a date-stamped record of the creation of the song. Should someone copy your song or a portion of the song, you would use this date-stamped envelope to prove that you had created the song first and, thus, own the copyrights (which include the rights to produce and reproduce the work).
There are a number of reasons why poor man's copyright is not a recognized method of protecting copyright for songs. Firstly, post offices do not consider themselves to be copyright registries and do not accumulate or store the information required by the courts to prove ownership of the song.
Additionally, they do not keep records of the registered mail for extended periods of time. Should someone copy your work in six years, it is doubtful the post office will have a record of when you sent your registration. As a result, it could be very difficult to prove whether you really sent the envelop through registered mail or whether the stamps were artificial.
There are other ways one could corrupt the registered envelop that also make poor man's copyright unsuccessful to protect your song. For instance, you could send yourself a number of unsealed envelopes just to get the date-stamp. Then, when you have completed a song, you could place it inside the envelop and seal it. In other words, you would be trying to pass-off a song for a date that is prior than the actual creation.
A further method of tampering with the copyright registration is sending yourself a sealed registration with your song. Then, if changes are made to the song, you could steam the envelop seal open and insert the updated version of the song.
As you can see, these cases reveal the ineptness of poor man's copyright to protect your songs and why the courts do not consider it a viable means of registration.
Fortunately, there are a number of copyright registries available to legitimately protect your songs. Whether it's through online registries, the government, or associations, there are various options available to protect your songs. Do your due diligence and pick a registry that suits your pricing and storage needs.