Please please find a more elegant way to represent this message to users.
Current application of this message takes up way more space in activity feed than at all justifiable for its value.
Horrible cluttering of the activity feed.. takes precious space that could otherwise show more of the email the user actually wants to skim/read.
Surely enough with a small warning icon next to the "# of messages in thread" icon with mouse-over message "Tracking data is not available for this email."