The correct answer is C because when an external domain publishes multiple MX records but only one specific host should actually be used for mail delivery, the clean administrative approach is to control that resolution internally through DNS. Proofpoint mail routing depends on the target destination the system resolves for delivery, and DNS is the normal mechanism used to determine which host should receive mail for a domain. Proofpoint’s own MX reference explains that MX records direct email to the appropriate mail server and that priority ordering controls fallback behavior.

If you simply let the mail system perform a normal DNS lookup against the public MX set, it may select among the published records according to priority and availability, which does not meet the requirement of forcing delivery to only one specific host. Likewise, using a wildcard does not create deterministic routing to the exact intended server. While directly entering a destination host in a route can sometimes be used in other routing contexts, the scenario here specifically involves controlling delivery for a domain whose public MX set does not reflect the desired operational target. Using an internal DNS override or internal DNS record lets the Proofpoint system resolve that domain to the exact host you need while preserving consistent routing behavior.

This aligns with the course emphasis on Mail Flow and routing control: when public DNS does not match the required delivery target, the administrator should use internal DNS to steer resolution properly. Therefore, C is the best answer.