An email administrator is setting up records for their new cluster of mail servers. What must each of their MX records point to?

Each MX (Mail Exchange) record must point to an A (Address) record, which is essential for the proper functioning of email delivery. The A record corresponds to the IP address of the mail server that will handle incoming emails for a particular domain. When an email is sent, the sending mail server queries the DNS for the MX records associated with the recipient's domain. The MX record then provides the hostname of the mail server responsible for receiving emails, which is resolved to an IP address through the A record.

This relationship is crucial because if the MX record does not resolve to an A record, the sending server will be unable to determine where to send the email. The A record is the fundamental building block for routing network traffic, making it indispensable for MX records to effectively direct email messages to their intended mail servers.

In contrast, while AAAA records serve a similar purpose for IPv6 addresses, CNAME records are used to create aliases for domain names, and PTR records are used for reverse lookups. None of these fulfill the necessary role of resolving the MX record to the actual IP address where email is directed.