SPF
also: Sender Policy Framework
SPF (Sender Policy Framework) is an email authentication protocol that allows domain owners to specify which mail servers are authorized to send emails on behalf of their domain.
SPF works by publishing DNS TXT records that list the IP addresses or hostnames of servers permitted to send mail from your domain. When a receiving mail server gets an email claiming to be from your domain, it queries the SPF record to verify the sender's server is authorized.
For example, if you own example.com, you might publish an SPF record like v=spf1 ip4:192.0.2.0 include:_spf.google.com ~all. This says Google's mail servers and the IP 192.0.2.0 are allowed to send mail from example.com, while other servers should be treated skeptically.
SPF is one part of email authentication, often used alongside DKIM and DMARC to reduce spoofing and phishing. It's widely deployed but has limitations—it doesn't work well with email forwarding, and some receivers ignore SPF entirely.