I previously asked if it might be possible to prevent people from using Yesware against you. The answer: Yes. Disable images -- especially for people you do not know. I'm not all that satisfied with this answer, as images seem to be an important aspect of the email experience (and even my trusted friends might be using Yesware to keep tabs on me).
Is there a way, perhaps via a Gmail extension or other application that can be used to track the trackers, to know if the Yesware pixel was served?
(Kind of like how Snapchat lets you know when someone has screengrabbed one of your snaps.)