Email represents an important part of the online advertising world because emails are a key factor in the online consumer-to-consumer communication. Anyone can get a free email account from email providers like Yahoo! or Google.

In the online world, turning your customers into your marketing force means having customers that will initiate and pass along positive email messages about your product or service to others. Consumers are influenced by information they receive from other consumers trough interpersonal communication like email. A person can communicate with a larger number of others, more quickly and more easily via email than many of the other channels. Passing along an email message is even easier than writing one's own comments. Furthermore, pass-along email seems particularly well-suited for the spread of images and/or verbal content that is too detailed to be spread in what has traditionally been known as word of mouth. However, email saturation and misuse (spam) have put a serious dent in response rates. Consumers are often quick to hit the delete key when they know the message is from a marketer. They are much more reluctant, however, to delete a message from a person they know.

Word of mouth is a form of promotion in which satisfied customers tell other people how much they like a business, product or service. Personal Influence: The Part Played by People in the Flow of Mass Communications , Katz and Lazarsfeld found word of mouth influence to be far more important than advertising or personal selling. The word of mouth conversation is the best feedback you’re ever going to get. It’s far better than any other kind of market research, because it is the authentic voice of the consumer.

Why do consumers pay attention to word of mouth? According to Schiffman & Kanuk in Consumer Behavior , the most basic motive for a consumer's attention to word of mouth messages is the expectation of receiving information that may decrease decision time and effort. A second motive for using word of mouth concerns the consumer's desire to decrease dissonant cognitions and increase cognitive consistency. Consumers want to reinforce their decisions or to increase their confidence in the views that they already have.

Online Consumer Psychology: Understanding and Influencing Consumer Behavior in the Virtual World by Curtis P. Haugtvedt, Karen A. Machleit and Richard Yalch contains edited versions of papers that were presented at the 2001 Advertising and Consumer Psychology Conference in Seattle, Washington. Published in 2005 the 576 page book has 23 chapters structured in 6 parts: Community, Advertising, Customization, Site Design, Decision Making and Research Tools and Approaches.