The email authentication standard that uses DNS TXT records to list authorized sending servers is SPF.
Sender Policy Framework lets a domain owner publish a policy describing which mail servers or IP addresses may send messages using that domain in the SMTP envelope-from address. Receiving servers retrieve the policy through DNS and compare it with the connecting server. A typical record begins with `v=spf1` and may list mechanisms such as `ip4`, `mx`, `a`, or `include`.
SPF was developed to reduce forged sender addresses, but it does not cryptographically sign a message. It also checks the envelope sender, which may differ from the visible `From:` address. That limitation is why DMARC is often used alongside SPF: DMARC checks alignment between authenticated domains and the address displayed to the recipient.
DKIM is the common mix-up. DKIM places a digital signature in the message header and publishes a public key in DNS, while SPF publishes an authorization policy. S/MIME instead provides end-to-end message signing or encryption for participating mail users.