Thanks for answering, it's curious enough that I'm now looking into Thunderbird plugins to see if there's one to block (or try to) tracking pixels.

Depending on the platform used to send the email, it's trivial to change that pixel to any other image, ie a brand logo in the template.

On the whole, disabling images entirely is the only real defence against email tracking.

The easiest way is to disable them by default which is why Thunderbird defaults to not loading from senders not yet whitelisted, which thankfully most HTML email has the text on the email, it's those darn emails that have all the text in an image that I get suspicious of and delete (usually spam / probably malware). It would be nice to see plugins to block them for sure. I use Thunderbird for work, keep forgetting to use it for regular email. At least even webmail email services block pictures by default now, probably to prevent browser exploitation, never know when someone finds some rogue PNG exploit.

You can't tell if an image is a 1x1 pixel until you load it from the server. At that point, the pixel has already done its job.

Most email clients these days don't block images by default unless it ends up in the spam mailbox.

They never embed images directly into emails either.