Does this even work for most clients, e.g. gmail and outlook? My understanding is gmail preloads the images (aka pixels) to their server before you even open. And outlook will cache the image after the first open so it won't fire back to the server after that.
I thought that too, but just looked talked with a user of an email marketing program and she is getting read notifications from her own Gmail account right after she opens it when testing (and lots in production). I think what Gmail does is they load the images from their server the first time you open the email and cache then for later views.
Working in a large corporation I can confirm that it doesn't work. Outlook blocks all images and external content by default. There is a button somewhere to unlock the email but you'd never click that for spam.
If they really cared, they'd made it simple to default to showing images from people in your contacts, and not otherwise.
However, because they're in bed with the advertisers, they make it so you either choose a degrading email experience for all your emails, or allow yourself to be tracked.
This comment misses the mark a bit. Gmail actually competes with spam emailers - then run their own ads inside your inbox and would rather you click on those. They also sort promotions and spam out of your primary inbox pretty robustly.
