Which of the following best describes DKIM?

DKIM, or DomainKeys Identified Mail, is best described as a mechanism that authenticates an email domain using a cryptographic signature, often referred to as a secret key. When an email is sent, the sender's email server generates a unique signature using this key, which is associated with their domain. This signature is then added to the email headers.

When the recipient's mail server receives the email, it checks the signature against the public key published in the sender's DNS records. If the signature matches, it confirms that the email was indeed authorized by the domain owner, thereby helping to prevent email spoofing and phishing attempts. This process ensures the integrity and authenticity of the email originating from the domain, which is why this choice accurately captures the essence of DKIM.

The other options do not accurately reflect what DKIM does. For example, verifying the sender's email address and checking email volume does not involve cryptographic methods or domain authentication. Measuring spam rates is also unrelated to DKIM's purpose, which is focused specifically on verifying the authenticity of the sending domain rather than assessing spam frequency.