Transactional email is a server-sent email triggered by a specific user action — like signing up, verifying an account, or resetting a password. One person did something, and they get one email in response.
There are two kinds of email an app sends:
| Type | Trigger | Example |
|---|---|---|
| Marketing | Sent to a list on a schedule | Newsletter, product announcement |
| Transactional | Triggered by one user's action | Verification link, password reset, receipt |
Marketing email goes to many people at once. Transactional email goes to one person because they did something.
Verification emails and password reset links are transactional. They need to arrive quickly (within seconds, not minutes) and reliably. Email delivery services like Resend are built specifically for this — fast delivery, high reliability, detailed logs showing whether each email was delivered, opened, or bounced.
Your app creates the email content (subject, HTML body with a link), hands it to an SMTP relay service (Resend), and Resend handles the actual delivery. The app doesn't send email directly — it delegates to a service that specializes in it.