Suppressions
A list of recipients the provider refuses to send to.
Suppression lists hold addresses that bounced, complained, or unsubscribed. Most providers manage suppressions automatically and refuse to send to suppressed addresses. Some let you import or export the list; treating suppressions as authoritative protects sender reputation.
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 suppressions, 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.