Ah I didn't see that one, they mention this on another page [0] (scroll a bit down). But they mention also that it still reliably detects views and that google released a new feature a while ago "dynamic email", which automatically loads but can also be disabled.
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Gmail does that too, no? I remember an HN discussion on this many years ago.
Here, I found a corroborating link from 2013: https://www.litmus.com/blog/gmail-adds-image-caching-what-yo...
Ah I didn't see that one, they mention this on another page [0] (scroll a bit down). But they mention also that it still reliably detects views and that google released a new feature a while ago "dynamic email", which automatically loads but can also be disabled.
[0] https://mailtrack.io/blog/google-email-tracking-gmail/