The plain email—which took no time to design or code—was opened by more recipients and had 3.3x more clicks than the designed email. The plain, unstyled emails resulted in more opens, clicks, replies, and conversions, every time. […] Replies to welcome emails were tripled. Cold emails were getting 30-35% open rates and 3% conversion rates, which is incredible.
Besides, every HTML email looks like an ad.