Short answer: send each welcome message once, poll delivery events outside the signup request, and turn every permanent bounce or complaint into a suppression decision before another send can be queued. For fintech onboarding, retain the event, the decision, and the message correlation ID as separate evidence. A dashboard saying "bounced" helps operations; it is weak evidence by itself. The least complex design uses four durable records: recipient state, outbound intent, raw provider event, a...