TL;DR: A series of markup and styling hacks that exploit HTML interpretation quirks of various web email services can be hacked to intentionally vary message appearance between services. Coupled with forwarding, which further transforms the email using service-specific quirks, you can make a game where different paths of forwarding across services trigger different appearances.
Fun hack! I feel like there should be some clever practical applications but I'm drawing a blank.
I wouldn't call that an anti-pattern, it prevents people you forwarded the mail to from easily unsubscribing you from the mailing list. They aren't subscribed, they shouldn't be able to unsubscribe.
> I feel like there should be some clever practical applications but I'm drawing a blank.
You could maybe make some sort of email analytics where you could guess the email client of the readers. Use a different background 1x1 gif image for each one then check your logs.
If you're in the email marketing business having to create and test email templates, this could actually be a very practical exercise and a team building game at the same time, I think. These bugs are a serious pain point sometimes for folks on deadlines. I could see making a sort of perpetual internal game out of this to make the job more fun and incentivize documenting new bugs found.
I remember reading Silicon Valley company being frustrated with whatever emails they had sent on company policy being forwarded to journalists (Kara Swisher??) . One hack they came up with was to slightly reword every email slightly to identify who did it.
Maybe - this solution would help in figuring out who the leakers are!
Fun hack! I feel like there should be some clever practical applications but I'm drawing a blank.