Transactional email
An email sent in response to a specific user action — receipts, password resets, shipping confirmations.
Transactional email is email sent in response to a specific user action — order confirmations, shipping notifications, password resets, account verification, payment receipts. The hallmark is one-to-one triggering: an event for a specific user causes a specific email to that user. The opposite is broadcast or campaign email, sent on a calendar to a list.
Transactional email has different deliverability characteristics than marketing email. Recipients expect it, engagement is high (people open shipping notifications and receipts), complaint rates are near-zero, and ISPs treat it preferentially. Many production email programs send transactional from a separate subdomain on a separate ESP (Postmark, Resend) to isolate reputation from marketing-side risk.
Transactional emails should be as plain and as fast as possible. Long delays between trigger and delivery erode trust (a "shipping confirmation" that arrives 3 hours after the package was already delivered is worse than no email). Marketing content in transactional emails (the "while you wait, check out our other products" pattern) crosses regulatory lines in some jurisdictions and erodes the trust that makes transactional email reliable.