> If you need the original email for legal/evidence purposes, you can back it up first, either through Gmail or Unattach.

Yes, that was covered in the other thread. Presenting a backup as evidence has a lot less strength than the original.

If the other person doesn't have the email (they deleted it) and you don't have the original because Unattach deleted it to replace it with a different copy then we're only left with an offline backup on 1 person's machine. Who's to say that offline backup wasn't tampered with (modifying headers, etc.)?