What are NS and A record checks used for?

NS and A records are two of the oldest pieces of the Domain Name System (DNS), the system that translates human-readable domain names into the IP addresses machines actually route packets to. For email people, they answer two very specific questions about an address you are about to send to.

NS records tell you which nameservers are authoritative for a domain. If you query example.com and get back ns1.cloudflare.com and ns2.cloudflare.com, you know who is in charge of answering DNS questions for that domain. If you get nothing, the domain has no nameservers assigned, which usually means it expired, was never fully set up, or is parked in a broken state. A domain with no NS records cannot resolve anything, including MX records, which means it cannot receive mail. Sending to it guarantees a bounce.

A records map a hostname to an IPv4 address (AAAA records do the same for IPv6). For email validation we are not usually checking the A record of the bare domain to deliver mail, that is what the MX record is for. But the A record still tells you whether the domain is being actively used for anything at all. A domain with valid NS records but no A record on www or the apex often signals a domain that was registered and abandoned. A domain whose A record points at a known parking IP (Sedo, GoDaddy, Bodis, Afternic) tells you the domain is for sale, not in business, and any address attached to it is almost certainly dead. The IANA-assigned parking ranges and known parking provider IPs are public, so a validator can flag them automatically.

Why this matters for a list you are about to send to:

• NS missing or NXDOMAIN. The domain does not exist as far as the public internet is concerned. Hard bounce on first send. Suppress.
• NS present, no MX, no A. Half-built or abandoned. Treat as undeliverable.
• NS present, A points at a parking page. The domain is in a holding pattern. Mailboxes on it almost never work, and if they do they often catch-all everything as spam traps.
• NS present, A and MX both healthy, domain resolves to a real website. The address is on a domain that is at minimum alive. Worth checking further (MX behaviour, SMTP probe, engagement history).

These checks are cheap, they run in milliseconds against any recursive resolver, and they catch a category of bad addresses that a syntax check (RFC 5321 format validation) will never catch. A joe@deadcompany.io is syntactically perfect. It is also a guaranteed bounce if deadcompany.io has no nameservers.

In practice we run NS and A checks as the first filter in any list hygiene workflow, before paying for an SMTP probe or a third-party validation API. There is no point spending money checking the mailbox on a domain that does not have a working DNS record. The same logic applies when you are measuring list quality on an existing file. If 4% of your addresses sit on domains with no NS records, that is 4% of your sends guaranteed to bounce, and a bounce rate above roughly 2% is what triggers reputation damage at the mailbox providers. NS and A checks are the cheapest insurance you can buy against that.