Great question. And yes... these are actually based in years of email marketing experience. My previous company used email prospecting as our primary customer acquisition strategy. We sent hundreds of thousands of emails each month and did all sorts of A/B testing to see what worked well and what didn't.
But... point well taken... none of that data is included here. It didn't fit the "tone" of the article, but perhaps I should have found a place to squeeze it in.