[jiveturkey]

When Gmail downloads the image it identifies itself as GoogleImageProxy, and will be coming from a GCP/Google ASN.

Similar signal will be there for any email provider or server-side filter that downloads the content for malware inspection.

Pixel trackers are nearly never implemented in-house, because it's basically impossible for you to do your own email. So the tracker is a function of the batteries-included sending email provider. Those guys do that for a living, so they are sophisticated, and filter on the provider download of images.