"Nicely designed" emails are things like flight confirmations, invoices, shipping notices, and that kind of transactional email. The common thread is that some entity with Real Money To Spend on a graphic designer is sending a (usually form) email to lots of people. Most spam outfits do not have those kinds of resources, especially since they need to change up their emails constantly to avoid spam filters.
The closest I would say that the "pretty" stuff gets to spam is newsletters and particularly political fundraising emails; I'm on a lot of political party lists after spending copious college free time working on campaigns.
And speaking of universal heuristic, in my experience, the quality and relevance of content is strongly inversely correlated with the quality of design. The prettiest websites out there are ones that deliver negative value. The best designed (according to modern trends) user interfaces are the ones with worst ergonomy, wasting user time the most.
And yes, the subset of spam that's most recognized - ED pills, reproductive organ enlargement, members of royalty looking for help managing their finances - they tend to be very simply designed. But their distinguishing feature isn't simplicity of design. It's the carelessness. Typos, bad grammar, highly visible formatting mistakes, etc. When, on occasion, one of that "old school" spam messages tries to pose as a legit transactional e-mail, you can see through the deception by noticing the carelessness in replicating the design of the company being impersonated.
This is just plain untrue, and seems to color your impression of email as well. See e.g. Delta vs Southwest (more polished look is more ergonomic site/app), or New Relic vs Datadog public-facing sites (indistinguishable in "quality" of the design, for competing products where IMO Datadog is better).
What are some examples that put the inverse correlation in your mind?
Indeed. The number of HTML elements in an email scales pretty linearly with the probability of it being spam.
Plain text is almost 100% certain not spam.
Spam usually has bad formating, mixed charset, old/broken layoyt etc.
Most of time, spam are odd designs. Example they only have HTML and didn't have the corresponding plain text part. They have large pictures and too little text, they have many "invisible" part so that hide behind element to trick you to click the wrong thing.
Even in plain-text, spam tend to include random links, very long text, weird chracter etc
A nicely design, or a plain-text emails(only plain text) with nice format all are good signal of non-spam.
In other word, consistency is signal of non-spam emails. But sometime marketing emails-which you never explicitly subscribe to, you just register for an account and got marketing emails, can also be consider spam, but from google/hotmail point of view they don't consider these are spam and that's reason they have Promotion tab to put thing in there
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Nearly 100% of spam is plaintext, a small fraction uses badly designed HTML that barely displays in the client.
If it doesn't have this link, it's spam. If it's trying to get me to buy something I didn't explicitly wishlist on a store, it's spam. If it's unsolicited marketing, it's spam.
And even if I were to include all marketing email, including those I am interested in, then plaintext mail would still constitute the majority of spam mails I receive.
Whenever I have to interact with a company, I check the spam folder for their replies.
The majority of mails in my spam folder is html mail.
Not all is as sinister as it looks.
The simply solution is to just can any senders that don't respond to unsubscribe requests. But those will be just as bad in plaintext as in HTML, so I don't see why Plaintext means it's not spam, they can do the same thing Plaintext.
I can recommend using the unsubscribe link if you forgot to uncheck the newsletter checkbox during registration. Alternatively send them a mail to A) remind them of the SPAM-CAN act or your local equivalent legislation and B) that they should delete your mail from the newsletter. That works almost always, if not you can always threaten legal action (where I live, that's an easy 600€ of profit in a courtroom).
I very rarely get marketing mails from any reputable company that are completely unsolicited. In most cases it's a followup from trying out an offering or updates to products I'm using or are adjacent to those. If I'm not interested, I either ignore them or unsubscribe if it's repeatedly not interesting.
This is a much more effective "spam" strategy compared to "all HTML is spam".
In my observations badly formatted spam exists, but have big intersection with the spam which is relatively easy to filter out. Nicely formatted HTML spam on other hand is hard to filter because the only difference with legitimate marketing email is lack of any consent from the sender. Sure I can hit an unsubscribe link and my be never will get spam from the same domain again, but spammer will know that this email is active and will include the address in spam send on behalf of other customers.
Among this sea of garbage, there are rare islands of useful updates - like genuine newsletters I subscribed to, or email from my doctor about test results, or reminder from my bank that they are going to deduct that car payment, whether I like it or not, so there better be money there, etc. And of course Amazon shipping notifications! If all those need any HTML, then only the most rudimentary one.
Of course, if you asked a user whether they'd want to get ugly marketing spam or beautiful one, I guess they'd choose the beautiful one. But that's not the right thing to ask.