I’d start by saying that of course your copy, audience targeting, timing and frequency are all important. It’s like asking “what part of your car is the most important to get it to drive from point A to B, the steering wheel, tires, or windshield?..”

Spoiler alert: it’s all of them, some more than others of course.

Hubspot has a good article on this here: 9 Critical Components for Optimized Marketing Emails, But, I also broke down the key components in my full answer here on Quora (click to read more).

P.S. For anyone already thrown off by the amount of effort that goes into perfecting an email campaign, we recommend Email Monks, a creative agency that works to understand your company and your target market.