Automatic Content Recognition (ACR) [42] is widely used for second-party tracking in smart TVs. As shown in Figure 1, ACR periodically captures frames (and/or audio), builds a fingerprint of the content, and then shares it with an ACR server for matching it against a database of known content (e.g., movies, ads, live feed). When the fingerprint matches, ACR server can determine exactly what piece of content is being watched on the smart TV.
If the mail is sent unencrypted the admin can read it. What I have is a script that encrypt incoming e-mail with the users key, so that they are stored encrypted on the harddrive. That at least protect against an intruder reading past e-mails. I use a Perl script written by Mike Cardwell for that.
Another service you might like to have for your users is WKD/WKS, so that senders clients can automatically fetch the public key for your users.