NEW YORK – Marketing via e-mail may be the next frontier. According to eMarketer's E-Mail Marketing Report, e-mail marketing is not only cheaper at less than one cent per message but can produce better quantifiable results than banner ads. The click-through rate for opt-in e-mail is 3.2% compared to .3% for banner ads. The low-cost is particularly attractive for small businesses which are estimated to spend $2.2 billion by 2005 on localized e-mail marketing, says The Kelsey Group. In its survey, 42% of small businesses indicted they will use e-mail for marketing by 2005. With eMarketer finding e-mail users outnumber Web users by 10%, and more than one billion e-mail messages being sent each day in the U.S., marketers plan on spending $2.1 billion on e-mail marketing by the end of the year. Roughly 80% of e-mail marketing messages receive a response within 48 hours, compared with six to eight weeks' response for traditional direct marketing.