> This seems like the worst part (not that the rest is not bad), anyone has an idea of what this "pixel trackers" are and how they actually work?

This is very old tech. "Invisible" image linked to a url. Pretty much every ad email has pixel trackers for data ( impressions, etc ). It's used heavily in marketing.

Not sure why google would use such primitive tech. There are better ways to verify that an message has been opened and read.