That's not quite true, in that all the emails I send to people who have opted in are now available to LinkedIn. You have no way of totally opting out without encrypting all your email.
For all you know, I (or anyone else you email) could have a simple "forward all email to root@nsa.gov" filter running. If you don't want the second party to share your email with a third party, don't send the second party an email.
