I somehow acquired a tracking mechanism on my Gmail which had 2 send buttons (blue icons) - one send & track, the other send & don't track at bottom left of the screen.. There was also an outline icon representing an envelope at top right of the screen. Clicking on the latter at any time gave a list of sent & tracked emails with white backgrounds for 'unopened' and green for 'opened. with details around this.
About a week ago, this facility disappeared as fast as it appeared and as I was used to it to track emails, I was upset! I have searched about 20 other trackers but no