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 developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to idempotency keys, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.