Mailing it may establish a date, but an envelope can always be opened after postmarking and resealed effectively without anyone knowing. Hence it is not a document that will hold up in a court of law. You don't prove copyrights by "reasonable doubt", you prove them with solid proof, like a registered copyright. You guys can debate this all you like, but my dad was a lawyer and I also asked a lawyer friend about this recently and she just laughed. It is a myth, plain and simple. Mailing a song, or a piece of art to yourself is NOT A VALID COPYRIGHT. You're doing people a disservice by insisting this is true when it is not.