Exponential backoff is the retry strategy of waiting longer after each failed attempt (1 s, 2 s, 4 s, 8 s…), used for 429 and 5xx responses so that retries do not amplify an overload.
When a request fails for a transient reason, retrying immediately and repeatedly makes things worse for everyone. Backoff spaces retries out, usually with some random jitter, and gives up after a fixed number of attempts. For 429 responses the server's Retry-After header takes precedence over the schedule.
Backoff only makes sense with idempotency: a retried send must not become a duplicate.
At SMSMeteor: the recommended policy is to retry 429 after Retry-After and 5xx with backoff from one second over five attempts, reusing the same idempotency key, and never to retry other 4xx.