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macOS Sonoma Mail bug: spam bypasses Block All Remote Content
Feedback Assistant Boycott Mail app on macOS has a privacy setting Block All Remote Content that prevents downloaded emails from connecting to the internet. For example, HTML emails frequently include image links, which can be used for tracking: when the image is loaded from a remote server, the owner of the server knows that you've opened the email! Block All Remote Content is supposed to prevent this kind of tracking, and it did… until macOS Sonoma.
However, I flagged the Reddit user's reply in Mail app for later, because I knew that I'd be updating from Ventura to Sonoma before WWDC in June, which indeed I did recently. After reading the old Reddit post again, I checked Little Snitch Network Monitor, and sure enough it showed a number of blocked outgoing connections to random domains other than my email providers. (By the way, Mail app has another bug that started with macOS Ventura: "There's no way to mark a message as Not Junk", which I reported to Apple a year ago as FB12217912 and still isn't fixed on Sonoma.)
Or read this on r/apple