Symptom: password resets and receipts land in spam right after a big marketing send, on the same account.
Cause: one account, one reputation pool. A high-complaint marketing blast poisons the engagement signals for the transactional mail sharing the identity.
Confirmation: compare engagement and spamreport rates per mail stream. If marketing's numbers are bad and transactional's delivery sagged at the same time, they are coupled.
Fix:
1. Create subusers for each stream (transactional, marketing) with their own API keys and, at volume, their own dedicated IPs or IP pools.
2. Authenticate the sending domains per subuser so the identities stay separate.
3. Monitor stats per subuser, not just account-wide, so a problem in one stream does not hide in the average.
Verification: a marketing complaint spike no longer moves transactional inbox placement, visible in per-subuser stats.