Sandbox mode
A starter mode that limits sending to verified addresses.
Some providers (most notably AWS SES) start every account in sandbox mode: you can only send to verified addresses until you request and receive production access. Plan for the approval lead time when launching.
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 sandbox mode, 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.