Email newsletter volume has been growing rapidly, and for several reasons. First, marketers noticed the higher open rates and click through rates versus straight, transactional messages (along with markedly fewer unsubscribes). It is a subtler exercise to place ads in margins of requested newsletters than blasting dedicated marketing messages. If the formula sounds familiar, it's because newspapers invented it 300 years ago.

A second reason is that newsletters better suit the relational role email is carving for itself. Email marketing began in a crude, promotional state - one in which spam persists today. But sophisticated marketers - with the help of backend CRM systems - have turned it into a customer communications tool as efficient at collecting information as sending it. Letting clients customize their own newsletters through dynamic personalization is as automated as it gets in updating customer databases.

Like most other media, email is first a publishing medium, and then a marketing medium. People don't obtain email addresses in order to receive marketing messages, anymore than they buy TVs or subscribe to magazines for that purpose. But once they have them, they prove receptive to the right message, aimed at the right person at the right time.

If you haven't already launched your customer newsletter, here are some general guidelines:

• Content - Think of the information customers most frequently want to know about your products and services. Also, think of the types of advice they most frequently seek from you.

• Layout - Keep your newsletter to a single screen and make it like the Contents page of a magazine. That is, a headline and short deck for each article and a Read More link that takes them to the full story on your Web site (where you can track who has clicked).

• Frequency - The ideal frequency depends ultimately on how useful you can make your information. Anything less than 6X per year is likely a waste. Most people simply receive too many messages to recall something arriving once every few months.

• Permission - Remember, it is an opt-in newsletter. It would be a little counter-productive to anger your customers by ignoring their privacy.