Yep. When Gmail first introduced this image proxy feature in 2013 it started showing images in emails by default, which is great. I researched blog posts from

Yep. When Gmail first introduced this image proxy feature in 2013 it started showing images in emails by default, which is great. I researched blog posts from then and apparently a workaround that still worked was to serve a fake HTTP Content-Length header of "0" and Gmail's proxies wouldn't cache the image. It's unclear if this bug has been fixed or not, or if similar bugs affect Outlook's proxies, for example. The rest of this post is speculation - I wonder if it won't affect Apple's Mail app because Apple isn't loading images directly from a proxy, instead, the original URL is sent to the Mail app over IMAP or Exchange and then Apple will download the image by asking the Apple proxy for the unmodified URL. This means even if an existing Gmail or Outlook image proxy server can be tricked, it shouldn't affect the Apple Mail app. That's not to say Apple Mail won't have other issues - for example, it shouldn't stop at images. Apple Mail supports CSS and web fonts, so theoretically all network traffic not destined to hit the IMAP server should go through the proxy if complete privacy is desired. I think the wording of the Mail app suggests it's more than just images. And the way it's implemented, because it's not server-side, it does indicate that an email address checked using Apple Mail downloaded your email, so you know it's pretty likely there's a human at the other end and they use Apple Mail even if they don't know exactly when you opened the email for the first time, they know when your Mail app downloaded it and possibly when you received a push notification about it. Unless it caches content with every request, which it might, you might also know how many different Apple Mail clients downloaded the message and when which might still indicate patterns of use especially if you can create a network of tracking pixels across different email messages. Finally, nothing about the feature actually anonymizes links or prevents specifically tracking pixels, but that's probably a good thing until we invent local Content Blocker extensions for Mail app, for example.