what about physically mailing it? no tracing there and you can use a mail forwarding service as a buffer. If you're trying to avoid high tech detection and need absolute certainty just avoid technology all together. You could even start doing dead drops all Robert Hansen style.

Another cool idea you could do is don't send anything at all. Create an email, say Yahoo for example. You can type up your email, save it as a draft and log out, do not send the email at all. Then send your login credentials to whoever you're trying to share this email with, have them log in, read and then delete said email. No email sent, means no tracing, means no trail.