A triggered email is an automatic response following specific actions. For example, a person fills out a subscription form on your website and then receives a welcome email shortly after. Or they purchase something on your website and you send them a receipt.
Because triggered emails are launched in response to an action, they are generally sent to individuals or small lists of contacts.
How do triggered emails work?
Traditionally you would use API calls to define the trigger launching associated email campaign. The technology behind it is the same, but now many ESPs have dedicated automation features which allow you to create complex triggered emails and programs in a user-friendly interface.