Don't design your emails
You can spend a lot of time to make emails look pretty, but it might be better to just don't style them at all. Greg Kogan did some A/B testing an concluded that sending plain emails results in more opens, clicks, replies, ...
Why are the plain emails crushing the performance of designed emails?
- They're less likely to be caught in spam filters. Having less HTML and fewer non-text elements such as images lowers the likelihood of triggering spam filters. You can use a free spam checker to validate this by testing plain and designed emails.
- 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.
- 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.