[buro9]

What I do know from measuring my marketing and transactional emails is that the least possible formatting results in the lowest probability that I end up in the spam folder.

The hypothesis I had here (unverified) is that using a tool to produce email makes your email look like spam produced by that tool (by other people). My hypothesis more specifically is that spam detection software weighs similarity in the structure equally to similarity in content.

My HTML emails have since been modified to be absolutely minimal HTML as hints to the layout and nothing else. They are more like plain text emails that have been polished with just the lightest sprinkle of HTML and CSS but nothing more.

The result of keeping things simple is that my deliverability is over 99% and the open rate also phenomenally high.

[aarondia]

When I receive an html email, I default to thinking there is nothing of value in it, and I don't read it. I use email as a more formal text messaging, or often group text messaging, system. Marketing newsletters, etc are just a clutter. But I do get a lot of them, so maybe they work for others.

[andrei_says_]

Are you aware of any studies verifying this hypothesis? Would be very useful in my team’s roadmap.

[citrin_ru]

Spammers sometimes (often enough to bother) are trying to fool spam filters by adding invisible or hard to see text copied from ham messages (e. g. it allows to bypass Bayes). To counter this spamfilters are trying detect and penalize invisible text in HTML emails e. g. <span style="font-size: 0px;">....</span>, display:none, text with the same color as background, e. t. c.

Running something like headless browser (Chrome) to parse HTML emails would require too much resources so spam filters use either simple ad-hock HTML parsers or just regexps. If you use complex HTML markup, e. g. lots of nested div/span tags, different CSS styles on different levels and global <style> block on top of all, they can be confused and will detect invisible text in a message where all text is visible (when CSS/HTML is rendered in Google Chrome; who knows how it would look in different mail clients).

If you use clean and simple HTML, there will be no leftovers like <span style="font-size: 0px;"> and you will not confuse spam filters.

[andrei_says_]

Ironically <span style="font-size: 0px;"> is a must for spacing tables if you want outlook compatibility.

[citrin_ru]

Ironically this technique was used in phishing for MS Office 365 credentials (users of which often use MS Outlook): https://www.avanan.com/blog/zerofont-phishing-attack

Rendering in Outlooks changes from time to time - it is worth to check if this workaround is still required.