More importantly, using a plain email would save lots of time and effort. As a goal-driven-lazy person, that’s a good enough reason to start experimenting.

They don't look like advertisements. The second the recipient interprets your email as an ad, promotion, or sales pitch—and it does take just a second—its chances of being read or acted upon plummet towards zero. A plain email leads people to start reading it before jumping to conclusions.

They feel more personal. It's no handwritten note, but it's much more personal than an over-designed email with the recipient's first name crammed somewhere inside.

The plain, unstyled emails resulted in more opens, clicks, replies, and conversions, every time.

They're less likely to go into the "Promotions" tab in Gmail (used by ~16% of all email users), for the same reasons above. From my testing, the plain emails typically end up in the Updates tab and some times even in the primary tab. Of course, the text in the email also affects this.

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.