I think it's routine to include a link to an image in every email. When the URL is loaded, they know you've opened it.

But this isn't reliable. People who block images don't record that they opened it. It's entirely possible that this accounts for "No one is opening those" privacy policy emails.

I thought most email clients block third party images by default? Certainly Outlook, Thunderbird, and K9 do. This must skew those results somewhat.

I think I saw statistics somewhere that most people read their mail via the web interface of their email provider, not via a client, and I don't know whether web clients block remote content loading.

I thought that might be the case.

I mean I block images but I also definitely ignore every "we're changing our privacy policy..." email.