Currently the default setting for a new account in Mail (K9) is that external embedded images are loaded and displayed automatically by default. So external (possibly personalized) URLs are being called which gives the sender the possibility to get information about the fact, that and when the recipient read the message.
Personally most of the time I’m interested in the text message or attachments but almost never in the external images of (mostly) advertisements.
K9 has the feature to disable auto loading images and then displays a button to load the images afterwards (if any are included) which – agreed – reduces ‘comfort’ a bit but gives a lot more privacy. Which is why I really think that auto loading should be disabled by default – following the slogan: “Regain your privacy!”
The mentioned setting is in “Account Settings” → “Reading mail” → “Always show images” → “No (Always display the Show pictures button)”
The latest stable K-9 release is 5.806 from 2nd September this year