Email marketing is used by e-commerce sites through sending offers and information about promotions to consumers who have made a purchase. Other times email lists are sold to businesses who send emails to target market consumers who may be interested in their product.
Email marketing in general refers to the mass emailing of customers or potential customers but it could be said that each email from a business is a potential marketing opportunity. Once an e-commerce retailer gets a new customer they will ask the customer in the ordering process if they are willing to receive promotions in the future by email. If a consumer chooses not to get future information the site is not allowed to market to them.
Sites may also ask if consumers would like marketing offers from third party suppliers as well. If a consumer agrees to receive marketing materials from third party suppliers their email address is added to a database that at times gets sold in bulk to other businesses for marketing purposes.