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 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 mta-sts, 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.