Email marketing is the act of sending a commercial message, typically to a group of people, using email. In its broadest sense, every email sent to a potential or current customer could be considered email marketing. It involves using email to send advertisements, request business, or solicit sales or donations. Email marketing strategies commonly seek to achieve one or more of three primary objectives, to build loyalty, trust, or brand awareness. The term usually refers to sending email messages with the purpose of enhancing a merchant’s relationship with current or previous customers, encouraging customer loyalty and repeat business, acquiring new customers or convincing current customers to purchase something immediately, and sharing third-party ads.
Transactional emails are usually triggered based on a customer’s action with a company. To be qualified as transactional or relationship messages, these communications’ primary purpose must be “to facilitate, complete, or confirm a commercial transaction that the recipient has previously agreed to enter into with the sender” along with a few other narrow definitions of transactional messaging. Triggered transactional messages include dropped basket messages, password reset emails, purchase or order confirmation emails, order status emails, reorder emails, and email receipts.
The primary purpose of a transactional email is to convey information regarding the action that triggered it. But, due to their high open rates (51.3% compared to 36.6% for email newsletters), transactional emails are an opportunity to introduce or extend the email relationship with customers or subscribers; to anticipate and answer questions; or to cross-sell or up-sell products or services.
Many email newsletter software vendors offer transactional email support, which gives companies the ability to include promotional messages within the body of transactional emails. There are also software vendors that offer specialized transactional email marketing services, which include providing targeted and personalized transactional email messages and running specific marketing campaigns
Direct email involves sending an email solely to communicate a promotional message (for example, a special offer or a product catalog). Companies usually collect a list of customer or prospect email addresses to send direct promotional messages to, or they rent a list of email addresses from service companies
Advertising, or permission marketing, is advertising via email whereby the recipient of the advertisement has consented to receive it. 
A common example of permission marketing is a newsletter sent to an advertising firm’s customers. Such newsletters inform customers of upcoming events or promotions, or new products. In this type of advertising, a company that wants to send a newsletter to their customers may ask them at the point of purchase if they would like to receive the newsletter.
With a foundation of opted-in contact information stored in their database, marketers can send out promotional materials automatically using autoresponders—known as drip marketing. They can also segment their promotions to specific market segments.