A company has publicly hosted web applications and an internal Intranet protected by a firewall. Which technique will help protect against enumeration?

Rejecting invalid SMTP email helps reduce spam and spoofing but does not prevent DNS-based enumeration of internal hosts.

Allowing full DNS zone transfers is the opposite of a protective measure - it exposes all DNS records to any requesting party, greatly aiding enumeration.

DNS enumeration allows attackers to map internal network topology by querying A records that resolve hostnames to IP addresses. By removing A records for internal hosts from public-facing DNS, those hosts become undiscoverable to external parties. This is a key split-DNS or DNS hygiene practice that limits the information available to an attacker performing reconnaissance against the organization.

Enabling null session pipes on Windows allows unauthenticated connections to enumerate shares, users, and groups, which increases attack surface rather than reducing enumeration risk.