What is Sender Policy Framework(SPF)?

Sender Policy Framework (SPF) is an email authentication protocol designed to prevent email spoofing and protect against phishing attacks. In its simplest form, it's a "list" of mail servers a domain owner authorizes to send email on behalf of a specific domain. The list and related information is stored in an SPF formatted TXT DNS record. Below is an example:

TXT @ "v=spf1 a include:spf.google.com ~all"

When a mail server receives an email, it checks for an SPF record to verify whether the sending server is authorized to send emails for that domain.