DomainKeys Identified Mail (DKIM) is an email authentication method that allows the person receiving an email to check that it was actually sent by the domain it claims to be sent from, and that it hasn't been modified during transit. It achieves this by adding a digital signature to the headers of the email, which can be used to verify the authenticity of the message.
DKIM is used to help protect against email spoofing, which is when attackers send emails that appear to be from a legitimate domain, but are actually sent from a different domain. By using DKIM, the recipient of the email can verify that the message was indeed sent from the domain it claims to be sent from, and has not been modified in transit.
To use DKIM, the domain owner must publish a public key in the domain's DNS records. This key is used to sign all outgoing email from the domain. When an email is received by the recipient, the recipient's mail server can use the public key published in the DNS to verify the digital signature on the email. If the signature is valid, the email is considered to be authentic and can be delivered to the recipient's mailbox.
In summary, DKIM is a way for email recipients to verify the authenticity of an email, and helps protect against email spoofing by allowing the recipient to check that the email was actually sent by the domain it claims to be sent from.