A systems administrator is working with clients to verify email-based services are performing properly.

Comprehensive and Detailed Explanation:

DomainKeys Identified Mail (DKIM) digitally signs outbound messages with the organization’s private key, enabling recipients to verify integrity and authenticity using the corresponding public key in DNS.

SPF validates sending server IPs, not message integrity.

DMARC builds policy enforcement on top of SPF and DKIM results.

TLS secures the transport channel, not the message content itself.