I've added comp.mobile.android to the list. Maybe you didn't know the group exists If this Pixel isn't an android phone, my apologies.
You could also add the Eudora group, which is mostly about Eudora, but that's an email reader so maybe they would have an answer.
I though of this, but now I don't think there is much relevance. Included anyhow: I've noticed, only with one email so far, but I haven't been checking, from a news website that includes advertising. I looked at the email and glanced at the ad, then closed the email and realized I wanted to click on the ad. I reopened the email and found a row of 3 ads but not the one I wanted. I thought it was part of the email!! I closed and opened the email two more times and the 2nd time for sure the row of ads had changed, so that means it changed the 2nd time too.
A big difference between paid adsand attachments made by the sender at the time of sending, although maybe not about this. Paid ads are added, not by my email server and not iiuc by the sender's email server??? but by something working with the sender. They, a link to them, are added long before it gets to my server, and the actual picture/ad is added, I guess when the email is opened. So the point is that in this case the embedded data, I think it's called, isn't sent to me until the email is opened.