While a business without any customers isn't actually a business, many startups spend too much money acquiring new customers (which is why customer acquisition cost is a key startup metric).

Yet, oddly enough, those startups often spend very little money retaining the customers they landed to continue an existing relationship so it pays dividends over the long term (which is why lifetime customer value is also a key startup metric). This baffles me since working to retain customers costs significantly less in money, time, and other resources.

But what if you're bootstrapping your startup and don't have significant sums to spend on acquiring new customers, much less retaining customers? Email marketing is one way to solve that problem.

But don't just take my word for it. According to one study, 91 percent of U.S. adults surveyed say they want to receive promotional emails at least once a month. And those emails work. According to another study, email marketing is 40 times more effective than Twitter and Facebook marketing combined.

As long as you market properly.

Which, in some ways, means not marketing at all -- at least in the traditional sense. Granted, if you're running a special promotion, a limited-time sale, a once-in-a-blue-moon offer, then by all means send a "marketing" email, especially to existing customers who may only need a small nudge to convince them to purchase more, or complementary, products.

But if the only goal of your email is to share your unique selling proposition (USP), your brand position, or the benefits of your products, in that case, an overt marketing message will typically fall on deaf ears. Do that often enough and not only will customers ignore your emails, they'll eventually unsubscribe.

A great example of email marketing is Inc. colleague Robert Glazer's weekly newsletter, Friday Forward. Robert started out writing a weekly note to his team. Team members often forwarded those notes to others who asked to be added to the list. Eventually Robert decided to make it a newsletter, and now the newsletter is read by over 100,000 people in fifty countries.

Friday Forward works because it's benefit-driven: each installment is intended to inform, educate, inspire; not sell. Although it has allowed Robert to spread the word about his upcoming book. That's the approach you should also take.

Create emails that help customers better use your products. That help them solve problems. That inform, educate, and inspire them. Do that, and they'll be much more likely to become repeat customers. Loyal customers. Even brand enthusiasts who help spread the word.

Don't focus on marketing your business. Focus on helping people, because people don't do business with companies. They do business with people.

Especially people who help make their lives better, even if just in a very small way.