What is SPF, DKIM and DMARC?
SPF
SPF (Sender Policy Framework) is an email authentication method designed to detect forged sender addresses during the delivery of the email. SPF detects a forged sender claim in the envelope of the email. Checking the FROM against SPF can be used to detect the forging of the visible sender in emails (i.e. email spoofing), a technique often used in phishing and email spam.
DKIM
DKIM allows the receiver to check that an email claiming to have come from a specific domain was indeed authorized by the owner of that domain. It achieves this by affixing a digital signature, linked to a domain name, to each outgoing email message. The recipient system can verify this by looking up the sender's public key published in the DNS. A valid signature also guarantees that some parts of the email have not been modified since the signature was affixed.
DMARC
DMARC extends two existing email authentication mechanisms, Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM). It allows the administrative owner of a domain to publish a policy in their DNS records to specify which mechanism (DKIM, SPF or both) is employed when sending email from that domain; how to check the From: field presented to end users; and how the receiver should deal with failures.
