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 SaaS product teams
A SaaS product sends on behalf of its customers, so the deciding factors are tenant isolation, whether suppressions can be scoped rather than global, and whether support can answer why one specific account did not receive one specific email. Lifecycle automation and transactional sending usually need to coexist.
Applied to idempotency keys, that means weighing multi-tenant support, event stream, the suppressions API, and operating track record ahead of the rest, against trial expiry notices, dunning, onboarding sequences, and per-tenant transactional mail.