A network engineer troubleshoots an issue with an SMTP relay server.

The engineer should implement a PTR record. A PTR record is used for reverse DNS lookup, which maps an IP address back to a hostname. This is the opposite of an A record, which maps a hostname to an IPv4 address. Mail systems commonly use reverse DNS lookups as part of anti-spam and mail reputation checks. If an SMTP relay server does not have a valid PTR record, receiving mail servers may reject, delay, or flag messages as suspicious.

The CompTIA Network+ N10-009 objectives include DNS record types, name resolution, email-related records, and troubleshooting network services. PTR records are specifically associated with reverse lookup zones and are the correct record type when the problem involves resolving an IP address to a domain name.

DNSSEC adds cryptographic validation to DNS responses, helping protect against DNS spoofing or tampering, but it does not create reverse DNS mappings. An MX record identifies mail exchangers for a domain, but it does not resolve an IP address back to a name. DoT, or DNS over TLS, encrypts DNS queries. Therefore, PTR is the correct answer.

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