The Electronic Frontier Foundation's Sydney Li and Bennett Cyphers explain how to stop people tracking you through email. Read-receipt beacons and other trickery abounds.
…third-party email tracking technologies will try to share and correlate your email address across different emails that you open, and even across different websites that you visit, further shaping your invisible online profile. And since people often access their email from different devices, email address leaks allow trackers (and often network observers) to correlate your identity across devices.
It doesn't have to be that way.
The nutshell: it's not enough to block remote images in the client anymore. But you're probably not even doing that. For many, many of you, here's the first step: