Apple Mail does it -- a piece in Wired. (paywalled, but they seem to allow a few views per month)
snip
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Email dates back to the ’70s, when computers couldn't display much in the way of graphics. Because of this, email protocols are more or less designed for simple text messages with attachments—which works until you want to add things like colors and images. By the ’90s, a workaround showed up: adding HTML code to an email message that points to images hosted on servers.
I bring this history up only because it's what makes modern email tracking possible. Most email newsletters you get include an invisible “image,” typically a single white pixel, with a unique file name. The server keeps track of every time this “image” is opened and by which IP address. This quirk of internet history means that marketers can track exactly when you open an email and your IP address, which can be used to roughly work out your location.
So, how does Apple Mail stop this? By caching. Apple Mail downloads all images for all emails before you open them. Practically speaking, that means every message downloaded to Apple Mail is marked “read,” regardless of whether you open it. Apples also routes the download through two different proxies, meaning your precise location also can't be tracked.
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and an older piece on tuning it on.