Transactional email
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 (such as customer referral programs).
Source: Wikipedia