Don’t talk down to people. I think this should be obvious. But looking at the email offers I get, clearly it isn’t. Don’t tell people, “Not buying this product would be idiotic!” or “Only a fool doesn’t understand the value in this service.” This whole email marketing strategy relies on you making people understand the value they can get from you. And sometimes that means making people see the downsides of not buying. But you can do that in a friendly, respectful way.
When it comes to deciding how to craft that perfect subject line, there appears to be really only one area to avoid: the subject line of 60 to 70 characters. Marketers refer to this as the “dead zone” of subject length. According to research by Adestra, which tracked over 900 million emails for its report, there is no increase in either open rate or clickthroughs at this 60-to-70 character length of subject line.