They can be, but they aren't required to be. An email that contains all the resources to display itself doesn't leak user information. An email that contains

They can be, but they aren't required to be. An email that contains all the resources to display itself doesn't leak user information. An email that contains a 1x1 tracking pixel, loaded when the receiver opens the email, lets the sender know that it has been received. In view of email spam, this is an unacceptable breach of privacy.