Transactional email
A message triggered by one person’s action — a receipt, a password reset, a shipping notice.
Transactional email answers something the recipient just did: bought, registered, reset a password, booked a table. It goes to one person at a time and is expected, which is why it gets read far more than anything else you send.
Marketing email, by contrast, goes to a segment on your schedule. The distinction is not cosmetic: consent rules, unsubscribe requirements and often the sending infrastructure differ between the two.
Because they are opened, transactional messages are worth designing properly — but keep the promotion light: a receipt that reads as an advert can move the whole stream into the marketing category in the recipient’s mind, and in some jurisdictions in law.