Tracker pixels are surprisingly commonly used by legitimate senders… your bank, your insurance company, any company you patronize. These assholes hide a 1-pixel image in HTML that tracks when you open your email and your IP (thus whereabouts).
I use a text-based mail client in part for this reason. But I got sloppy and opened an HTML attachment in a GUI browser without first inspecting the HTML. I inspected the code afterwards. Fuck me, I thought… a tracker pixel. Then I visited just the hostname in my browser. Got a 403 Forbidden. I was happy to see that.
Can I assume these idiots shot themselves in the foot with a firewall Tor blanket block? Or would the anti-tor firewall be smart enough to make an exception for tracker pixel URLs?
I suppose you could even say text-based clients are at a disadvantage because when we opt to render the HTML graphically, a full-blown browser is launched which is likely less hardened than something like whatever profile and engine Thunderbird embeds.
In my case I created a firejailed browser with --net=none so I could hit a certain key binding to launch the neutered browser to render an HTML attachment in a forced-offline context— but I was too fucking lazy to dig up what keys I bound to that which is why I (almost?) got burnt.