MTA-STS
A policy that tells mail servers to require TLS for inbound delivery.
Mail Transfer Agent Strict Transport Security lets a domain publish a policy requiring encrypted SMTP connections from supporting senders. It helps prevent downgrade attacks where delivery falls back to unencrypted transport.
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 mta-sts, 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.