Financing: A newly acquired subsidiary of the advertising giant Omnicom Group, Innovyx doesn't share its numbers. But Harding said the company has been profitable.

Who partakes: Innovyx lists several A-list clients, including Sony, Adobe, Toyota and Hyatt. The company manages Hyatt's Gold Passport program, where customers can choose to receive their travel statements by regular mail or electronically. By selecting the latter they can read and sort statements anywhere they may travel. Innovyx also supplies travel offers and information related to the customer's travel habits.

How spam fits in: Harding said the most important aspect of e-mail marketing is permission of the recipient. That sets it apart from unwanted spam. At the same time, mail providers often block images, degrading the experience of an effective marketing message. There's also fickle reactions among recipients.

"People get mad at spam and companies get caught in the crossfire," Harding said. "They can't lash out at the person who sends a bad stock tip so they direct their anger at whoever sent the last message. It makes the environment trickier."

E-mail vs. TV: Will an e-mail marketing campaign ever generate the buzz of a popular television ad?

"It's already happened," Harding said. "Companies have already provided original content in their e-mail marketing messages. People respond to them and pass them around to each other because they are cool and fun. And in some cases, the amount of messages that have been passed around exceeds what was sent out in the first place."

— Charles Bermant