SPF
A DNS record listing which servers may send for your domain.
Sender Policy Framework is a DNS TXT record that names the IPs and hostnames authorized to send mail for your domain. Mailbox providers check the senders IP against the SPF record. Misconfigured SPF is a common cause of mail going to spam.
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 spf, 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.