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 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 one-click unsubscribe, 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.