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 previous customers and to encourage customer loyalty and repeat business,
  
• sending e-mails with the purpose of acquiring new customers or convincing current customers to purchase something immediately,
  
• adding advertisements to e-mails sent by other companies to their customers, and
  
• sending e-mails over the Internet, as e-mail did and does exist outside the Internet (e.g., network e-mail and FIDO).

Researchers estimate that United States firms alone spent US$400 million on e-mail marketing in 2006