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.
