SPF (Sender Policy Framework)
A DNS TXT record listing which servers are allowed to send email on your domain's behalf.
SPF is the original sender authentication standard. The record (a TXT entry on your apex domain) lists IP addresses or hostnames authorized to send email "from" your domain. Receiving servers check the message's SMTP envelope against this list — if the sending server isn't listed, the message fails SPF.
A typical BoomSauce SPF record looks like `v=spf1 include:_spf.boomsauce.com ~all` — meaning "trust everything in our SendGrid SPF chain, soft-fail anything else."
SPF on its own is weak (it can be bypassed via header forgery), which is why DKIM and DMARC are the rest of the stack. But missing or broken SPF puts a hard floor under deliverability — Gmail and Outlook downgrade unauthenticated mail aggressively.