The "mail it to yourself" trick might be evidence if you have to go to court to defend your copyright, but these days, there are better ways to prove you wrote something when. Like the file creation date on your Word doc. (Which isn't 100% either, but is no more spoofable than an envelope.)
Mailing a doc to yourself never had a particular legal value; it was just one way of countering a claim of "who is this author who claims he wrote this story that our staff writers produced independently as a work for hire?" It wasn't definitive, just a piece of evidence an author could bring to court. These days, the author's more likely to have timestamped emails to his friends describing progress on the novel.