DKIM is email-authentication technique similar to SPF. DKIM allows the receiver to check that an email was indeed sent and authorized by the owner of that domain. This is done by adding a digital DKIM signature on emails. A DKIM signature is a header that is added to a message and secured with encryption.
Authenticating legitimate sending sources with DKIM gives the receiver (receiving systems) information on how trustworthy the origin of an email is, and it can significantly improve the overall deliverability of an email channel.

In practice, DKIM on its own is not enough to fully protect an email channel. The email validation system DMARC is often mandatory and required for compliance as it creates a link between SPF and DKIM by validating whether a sending source has been authenticated with either SPF or DKIM.
Furthermore, DMARC allows organizations to instruct email services like Gmail, Hotmail and others to reject all emails that are not aligned with SPF and or DKIM.
