One-click unsubscribe
A standards-based unsubscribe action in the inbox UI.
One-click unsubscribe uses List-Unsubscribe and List-Unsubscribe-Post headers so supported mailbox providers can show an unsubscribe control directly in the inbox. Gmail and Yahoo require it for many bulk senders.
reading this as teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to one-click unsubscribe, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.