Use a durable delivery-claim ledger keyed by the logical password-reset challenge, channel, and message purpose; commit that claim before dispatch, reuse the same idempotency key on every retry, and record an ambiguous outcome as unknown rather than pretending the notification was sent exactly on...
Source: [Dev.to](https://dev.to/caderaven6851/password-reset-event-notifications-idempotency-keys-for-email-and-sms-retry-audits-20dm)