First, it's important to understand that e-mail now occupies a special place in the portfolio of marketing channels. It is exceptionally measurable and highly targetable, even at the individual level. It is usually the most cost effective channel, the one with the highest ROI.

However, it is also among the channels with the narrowest reach, as compared to online ads, search, mail or television.

Companies still struggle to maintain valid e-mail lists on more than 50 percent of their customer base, and many fall far short of that. In short, e-mail is a great way to communicate with your brand's core online audience.

But integration is a top priority for many marketers right now.

Companies must figure out how to assemble data from external and internal sources to build that elusive single view of the customer. Here are three flavors of e-mail integration.

Expanding messages delivered in other channels to e-mail

This approach is probably the oldest of the three, usually paired with direct mail. There has been evidence for some time to suggest that two related messages delivered in the same time window, one in email and one in direct mail, will drive better performance for both messages than either would achieve alone. But marketers would be wise to test carefully here and consider all options given the additional effort required to coordinate messages.

Extending e-mail messaging to other channels

This technique is newer. Examples include sending an e-mail with a special offer then following up with a mobile message referencing the offer. Or follow up with a physical mailing, but only to those who click on the offer and whose other characteristics make them likely responders.

Have your Web site reference an offer on the home page for customers who opened or clicked on an e-mail. The technology to accomplish all these message extensions is available, although these techniques are not yet broadly adopted by marketers.

E-mail in a more complex communication stream

This technique is my favorite, and the one I expect will someday be the dominant use of e-mail communication. You just visited the Web site, lingering over a particular product. Should you not receive an e-mail that in some manner builds on this behavior? “Time to renew.” “Your warranty will soon expire.” “A new store is opening in your area.” Connect the customer with the brand, and use email to continue the conversation followed by an offer of some benefit. When customers receive pertinent, timely information that extends their relationship with a brand, their attentiveness to future e-mail from that brand increases.

For all of the integration techniques discussed, the critical factor making everything possible is data integration across multiple channels. This challenge is most acute for campaigns that are continuous, not one-off, and based on triggered behavior which also happen to be the type with the greatest potential to drive results.