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 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 spf, 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.