Email marketing is the practice of sending marketing messages to prospective and current customers via email in order to sell, educate, or build loyalty.
Email is an “owned” digital marketing channel —that is, the sender fully controls the content and distribution— and typically works best when used to send personal, relevant messages to segmented lists of recipients. Email is an especially important tool in relation to ecommerce, where it’s used for sending transactional, promotional, and lifecycle messages.
source