Privacy-oriented email providers tend to block remote images by default[1]. Ad companies that provide free email services generally do not, because the users of tracking pixels are often their advertising customers.
[1] Fastmail also has a "middle-ground" option to show remote images if it's from someone in your contacts, if you want a way to filter it dynamically a bit. I leave mine on a straight block, but if you really hate seeing "ugly" emails, it's a nice option.
[1] Fastmail also has a "middle-ground" option to show remote images if it's from someone in your contacts, if you want a way to filter it dynamically a bit. I leave mine on a straight block, but if you really hate seeing "ugly" emails, it's a nice option.