Having had that a few times, and recently relaxing Firefox's referer policy for some other site compatibility which had avoided it before, I came to this workaround with uBlock Origin:
Oh sorry, I did not know that was the guy who hates being linked to from HN. Copy the link manually to read it if you still care to learn what he wrote.
Well, HTML e-mail arbitrarily stylable by the author was a mistake. This could have been avoided by a profile of HTML that removes presentation markup and CSS, leaving only pure semantic markup that a client can render in a "reader mode" equivalent.
It's not so bad. You can live with it. Being CREATIVE and expressing your personality damages the medium and impairs conveying your message. So: don't.
Huh. OK, then. Interesting datapoint. It was a PITA flop in the UK -- it occasionally kicked in when an SMS went over length or you added a smiley, and cost you 10x as much to send.
Some sites don't even bother anymore and just send the contents as a multi-MB image. Sort of "pdf as email" to not bother checking if the template works for all sorts of mail clients.
https://winworldpc.com/product/microsoft-outlook/97