Message class is a small field with large consequences. It decides three separate things: whether the recipient needs to have consented to a topic, whether frequency caps apply, and which sending route the message takes.
The two classes
Marketing is anything promoting something. It requires topic consent, it is subject to frequency caps, and it must carry an unsubscribe mechanism.
Transactional is a message the recipient’s own action triggered and expects: a receipt, a password reset, an order update, a booking confirmation. It is not gated on marketing consent, it is not frequency-capped, and it typically does not require an unsubscribe link.
Where people get it wrong
The temptation is to class a marketing message as transactional to bypass consent and capping. It works, briefly. It is also the fastest way to generate complaints, because a recipient who did not opt in and cannot easily opt out has one remaining option — and it is the expensive one.
The honest test: would this message exist if the recipient had not just done something? A shipping notification passes. A shipping notification with three product recommendations underneath it does not.
Why the classes should route separately
Beyond consent, class should determine infrastructure.
Marketing mail is bursty and its complaint rate is structurally higher. Transactional mail is latency-sensitive — nobody waits patiently for a password reset. Sharing a route means a promotional send can queue ahead of a reset, and the marketing complaint rate affects the reputation delivering it.
Separate provider connections and separate sending routes per class solve both. Several providers make this easy by offering distinct message streams that map directly onto the same split.
Across several brands
Class interacts with brand. A contact might be transactional-only for one brand — they bought something once — and fully marketing-consented for another. Holding class and consent per brand rather than per contact is what makes that expressible without either over-mailing or under-serving them.