hands private keys to the signing MTA, and publishes public keys on the DNS. The DNS labels are structured as selector._domainkey.example.com, where selector identifies the key pair, and _domainkey is a fixed keyword, followed by the signing domain's name so that publication occurs under the authority of that domain's ADMD. Just before injecting a message into the SMTP transport system, the signing MTA creates a digital signature that covers selected fields of the header and the body (or just its beginning). The signature should cover substantive header fields such as From:, To:, Date:, and Subject:, and then is added to the message header itself, as a trace field. Any number of relays can receive and forward the message and at every hop, the signature can be verified by retrieving the public key from the DNS. As long as intermediate relays don't modify signed parts of a message, its DKIM-signatures remain valid. DMARC DMARC allows the specification of a policy for authenticated messages. It is built on top of two existing 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; how the receiver should deal with failures - and a reporting mechanism for actions performed under those policies. Other methods A range of other methods have been proposed, but are now either deprecated or have not yet gained widespread support. These have included Sender ID, Certified Server Validation, DomainKeys and those below: ADSP ADSP allowed the specification of a policy for messages signed by the author's domain. A message had to go through DKIM authentication first, then ADSP could demand a punishing treatment if the message was not signed by the author domain(s) —as per the From: header field. ADSP was demoted to historic in November 2013. VBR VBR adds a vouch to an already authenticated identity. This method requires some globally recognized authorities that certify the reputation of domains. A sender can apply for a reference at a vouching authority. The reference, if accepted, is published on the DNS branch managed by that authority. A vouched sender should add a VBR-Info: header field to the messages it sends. It should also add a DKIM signature, or use