Like this one, maybe: Apple Mail Now Blocks Email Trackers

I like that Pegasus Mail does that, if you stick to plain text versions. It humors me when I receive a notice from my bank or an online store asking me to update my email address because they think that I am not receiving their emails. They would know better if they relied on failed delivery notices rather than trackers.

IER doesn't download these "beacons" even if image download is enabled, but anyway: As soon as you download any image at all they can track you - not to forget about clicking links in emails (which isn't recommended either for safety reasons): Lots of these are redirected via tracking sites, most likely the one mentioned in this thread's subject title - they don't even try to really hide it as you can tell by looking at the URL-Info tooltips IER provides ...