These results seems quite suspicious to me. In all instances the author only received 1 email in reply. If you run these results through an A/B testing calculator, they aren't statistically significant. That's even the case for the 1% conversion compared to the shortest email 8% conversion. I like the thesis statement that short emails convert users better, but the experimental results certainly aren't strong enough to support that claim. I'd love to see an updated version of the article now that 3 years have passed and see what the conversion rate is.