Two plus two equals four, right? Not always. When it comes to e-mail, a relatively tiny investment can net huge returns -- or disappear down a black hole forever, taking any potential future returns with it.

The first thing an e-mail marketer must do in this strange new world is to forget the marketing axiom about success being a matter of reach and frequency. Foisting your messages on as many people as possible, as often as possible, simply doesn't work in e-mail. So why, according to repeated industry findings, are so many marketers still doing it?

One reason: e-mail's crazy math. Compared to traditional direct marketing methods, e-mail is disarmingly cheap, which can lead to short-term tactical thinking and negligible oversight. Before e-mail became popular, companies would ask, "How often can we afford to communicate with customers?" Now they should ask, "How often is too often to contact customers?"

More is less, less is more
If you shoot a thousand arrows you're bound to hit a few bull's-eyes. If you send few million e-mail messages, someone is bound to respond. But sending too often leads to the phenomenon of the incredible shrinking list. Unless your customers are so highly engaged that they want to hear from you as often as possible, the more e-mails you send, the more they will opt-out, stop reading messages, or finally hit the "spam" button.

The following conversation is all-too-common in e-mail. Perhaps you've even had it yourself:

Marketing executive: "What kind of results did we get on last month's campaign?"
E-mail marketer: "We ended up with $200,000 in additional revenues."
Marketing executive: "Great! Next month, let's send it out four times."

Just because a campaign produces a certain amount of money in one month does not mean it will produce four times as much if you send it four times in a month. Not only does over-sending fail to deliver linear results, it can easily lead to decreased response rates that will drive your overall results below the levels of single monthly campaign. Worse yet, over-sending can actually work for short periods -- while silently ravaging future response and goodwill toward your brand. Never forget that customers will give you the benefit of the doubt just up until the point where they disregard every subsequent e-mail you send.

If you're really tempted to send more often, segregate a small portion of your list and increase message frequency just to those recipients. Measure response over three or four months. At the end of that time, you should have a clearer idea whether you can safely increase frequency to the rest of your list -- or thank your lucky stars that you didn't.

An e-mail equation
If you get a message from your mother, can you find that message amid the clutter of your inbox? Most people immediately say "yes." This is because the people whom you know and trust have a high E-mail Brand Value, or EBV. You are able to find your mom's message because her name has value to you. The formula for EBV is:

EBV = existing brand (+/-) relevancy + content value - frequency

To be clear, this formula is more directional than mathematical. But EBV can help you understand how your customers perceive your company in the online world, and how that online perception affects your overall brand. I discuss this powerful concept and assessment tool at length in my book, "The Quiet Revolution in E-mail Marketing."

Just like your mother's messages, you can make your company name have so much value to your customers that they're willing to open your messages based solely on seeing your name in the "from" field. Conversely, if you focus on short-term acquisition and promotion at the expense of relevance you will sacrifice most, if not all, of your positive brand value.

The sum of it all
The truth is, you don't really need to forget everything you know. But to succeed in the new world of e-mail marketing, you need to move away from the old assumptions and adopt a new way of thinking.

When it comes to the highly personal space of the e-mail inbox, 90 percent of U.S. Internet users say they want control over what people send them, according to October 2004 figures by TRUSTe and Taylor Nelson Sofres. Moreover, surveys by JupiterResearch consistently find that high degrees of relevance boost return, while increased frequency degrades response.

In an August 2005 report titled, "The ROI of E-mail Relevance," JupiterResearch noted that the most highly targeted e-mail campaigns can produce up to nine times the revenue and 18 times the profit of broadcast mailings.

The good news for small- to medium-size businesses is that technological advances have made it possible for anybody to send appropriately-timed and meaningful messages. While the largest companies have a lot to lose by doing e-mail wrong; smaller enterprises have a unique opportunity to build a fabulous e-mail brand from the ground up.