No it isn't. The email passes through the server unencrypted (between Postfix and Dovecot), so it's still a potential tap point. In fact, running encryption to the server simply says to the NSA: Ask Linode for access to this disk.
He used an encrypted FS I believe, but the NSA could just ask to monitor all mail to and from his instance at Linode (or any ISP) since he didn't encrypt the mail.