> - obviously you can encode some ID into the image URL itself but all that lets you do is identify the email address of the user that opened the email. But you

> - obviously you can encode some ID into the image URL itself but all that lets you do is identify the email address of the user that opened the email. But you already had their email address because you sent them an email - so again, no PII gets disclosed This functions as a read receipt (like the tracking pixels). The image might be cached later, but since it is initially loaded the first time an email with it is is opened, this means that implementing read receipts on all outgoing emails is as simple as making the URL for each image unique to the user. Thus, the marketer knows: 1) That the email was opened 2) When the email was opened along with whatever information they already have about the user. This is a HUGE privacy implication. Even if "no [further] PII gets disclosed", it discloses a lot of information that is both sensitive and easy for marketers to join with existing identifying information.