|DomainKeys Identified Mail (DKIM) is an open standard email authentication specification. The idea originated in around 2005. In simple terms, it allows the sending server to cryptographically sign (and take responsibility for) a message. The receiving server can then validate the signature by comparing it to a public key stored on the sending server's domain.
Sender Policy Framework (SPF), which preceded DKIM by around 2 years, has the same objective as DKIM, but is based on IP addresses and the premise that only domains specified in a server's public DNS SPF records are allowed to send mail using the sender's IP Address.