Idempotency keys
A header that prevents duplicate sends on retry.
An idempotency key is a unique value the client sends in a header. The server caches the response and returns the same response for any request with the same key, even if the network retries. Critical for transactional sends where a duplicate password reset email is at minimum confusing and at worst a security issue.
reading this as teams building autonomous agents
An agent stack treats email as a two-way primitive. Inbound has to arrive as structured JSON a tool call can consume, sends have to be safe to retry when a model loop repeats itself, and every action needs an audit trail. A provider without inbound parsing only solves half the problem.
Applied to idempotency keys, that means weighing inbound parsing, idempotency keys, webhook coverage, and operating track record ahead of the rest, against inbound parsing into tool calls and model-generated sends that must not duplicate.