Advertisers often embed links to their content in HTML emails. These may be pictures, advertisements, etc. These links are used for tracking.
The Mail app currently follows these links. I receive “breaking news” updates from a local newspaper, and these emails embed ads that are loaded each time the email is opened in Mail. I know this because the ads change from time to time.
Many modern email clients have an option not to download external content unless specifically instructed. This prevents tracking.
The /e/ Mail app does not have such an option (at least, not an obvious one that I can find). In the interest of privacy, such an option should be added and default to not downloading external content.