I'm curious if there's any precedence in the courts. But as far as convincing a judge or jury one way or another, it's some evidence that can help you out in a "he said she said" case. Printed out emails are frequently used in civil cases to help convince the judge of one thing or another, and they're quite easily forged but that doesn't stop them from being considered.

Other methods include putting it in public dirs, pastebin, etc. and letting the internet mirror it for free. Encrypt it if you must, torrent an encrypted tarball if you've got name recognition..