https://www.schneier.com/blog/archives/2017/10/e-mail_tracking.html

E-Mail Tracking

Interesting survey paper (https://senglehardt.com/papers/pets18_email_tracking.pdf):
on the privacy implications of e-mail tracking:

    Abstract: We show that the simple act of viewing emails contains privacy pitfalls for the unwary. We assembled a corpus of commercial mailing-list emails, and find a network of hundreds of third parties that track email recipients via methods such as embedded pixels. About 30% of emails leak the recipient's email address to one or more of these third parties when they are viewed. In the majority of cases, these leaks are intentional on the part of email senders, and further leaks occur if the recipient clicks links in emails. Mail servers and clients may employ a variety of defenses, but we analyze 16 servers and clients and find that they are far from comprehensive. We propose, prototype, and evaluate a new defense, namely stripping tracking tags from emails based on enhanced versions of existing web tracking protection lists.

Blog post on the research: https://freedom-to-tinker.com/2017/09/28/i-never-signed-up-for-this-privacy-implications-of-email-tracking/