Separating sending identities is not a cosmetic choice — it is the entire point of this capability, and skipping it undoes the protection almost completely. If marketing and transactional mail share the same domain or IP reputation, a complaint spike from one bad promotional send degrades deliverability for both at once, meaning the scenario this feature exists to prevent — a confirmation delayed by an unrelated newsletter's problems — happens anyway despite the feature being present. A dedicated subdomain, kept genuinely separate in how mailbox providers see it, actually delivers isolation, not merely labelling a message 'transactional' internally while it shares infrastructure with campaigns.
Suppression scope here is deliberately narrower than marketing suppression, and getting the distinction right matters in both directions. A customer who unsubscribed from promotional mail specifically has not withdrawn consent to receive a receipt for a booking they just made — that receipt is a service communication tied to a transaction they initiated, not a promotion they can decline like the newsletter. But a hard bounce or full account-closure request still has to suppress transactional sends too, since an invalid address or closed account should not keep receiving any mail, transactional or otherwise.
Because these sends are triggered by a specific action and bypass the batching pipeline entirely, they need to be near-instant and reliable in a way a scheduled broadcast does not — a confirmation arriving late reads as a system that might have failed, even once it arrives. This makes transactional delivery a different job, prioritising speed and per-message reliability over the batching a large broadcast depends on.