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 early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to spf, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.