E-mail marketing is a form of direct marketing which uses electronic mail as a means of communicating commercial or fundraising messages to an audience. In its broadest sense, every e-mail sent to a potential or current customer could be considered e-mail marketing. However, the term is usually used to refer to:
Sending e-mails with the purpose of enhancing the relationship of a merchant with its current or old customers and to encourage customer loyalty and repeat business.
Sending e-mails with the purpose of acquiring new customers or convincing old customers to buy something immediately.
Adding advertisements in e-mails sent by other companies to their customers.
Emails that are being sent on the Internet (Email did and does exist outside the Internet, Network Email, FIDO etc.)
Researchers estimate that US firms alone spent $400 million on e-mail marketing in 2006.