Glossary
Idempotency key
Also called Idempotency-Key header.
Last reviewed 18 September 2026
What is an idempotency key?
An idempotency key is a value a sender attaches to a request so that a provider receiving the same request twice performs the work once, which is what makes retrying after a timeout safe rather than reckless.
It matters in sending because the dangerous failure is not an error, it is silence. A request that times out may have been received, acted on and lost on the way back, so a naive retry delivers a second copy of a lesson to somebody who already has it.
The key has to be derived from the thing being sent rather than generated per attempt, and it has to be byte-identical on every attempt forever, or the second attempt is simply a different request. A row identifier is the usual choice, because it exists before the first attempt and never changes afterwards.
See also
- Throttling
Throttling is a receiving provider deliberately slowing or temporarily refusing mail from a sender, usually with a 4xx response, because the volume, the pattern or the reputation it is seeing is more than it is willing to accept right now.
- Soft bounce
A soft bounce is a temporary refusal of a message by the receiving server, carried in a 4xx SMTP response, because the mailbox is full, the server is greylisting you, or you are being rate-limited.
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