Cold email operators obsess over bounce rate for good reason: mailbox providers read a high hard-bounce rate as proof of a scraped or stale list, and reputation follows. The standard fix is email verification — checking whether the individual mailbox exists. But there is a layer above the mailbox that verification tools sometimes gloss over: whether the domain itself is capable of receiving mail at all. A surprising number are not.
As of the 2026-07-05 snapshot of our daily OpenINTEL-based scan, 664,715 Tranco top-1M domains publish MX records — and within them sits a museum of configurations that can never accept a message:
| MX target | Domains | What happens to your email |
|---|---|---|
| Empty target (blank hostname) | 4,925 | No server to connect to — immediate or deferred bounce |
localhost | 502 | Your own sending server tries to deliver the mail to itself |
~ | 157 | Not a valid hostname; resolution fails, mail queues then bounces |
mail (no dot) | 253 | A relative name that resolves against the wrong zone, or nowhere |
Nearly six thousand domains in the world's most-visited million, publicly announcing mail servers that do not and cannot exist. Some are parked domains with copy-pasted zone files; some are typos that nobody noticed because nobody at the company reads mail on that domain; some are half-finished migrations. From your side of the SMTP connection, the cause does not matter — the bounce is identical.
There is a right way to refuse mail — most of these are not it
A domain that genuinely wants no email — a CDN domain, a link-shortener, a brand-protection registration — has a standard, RFC-blessed way to say so. RFC 7505 defines the Null MX: a single MX record with preference 0 and a root target.
no-mail-here.example.com. MX 0 .A sending server that sees 0 . knows instantly and authoritatively that mail is not accepted: it can reject at submission time with a permanent error, no connection attempts, no retry queue, no five-day timeout. The localhost and empty-target hacks produce the opposite: your MTA tries, fails, queues, retries, and finally hard-bounces — slowly, and in a way that looks like a delivery failure rather than a policy. We cover adoption of the standard, and the hacks people use instead, in our Null MX field guide.
What this means for your bounce rate
For an outbound operation, these domains are pure downside, and they are trivially detectable before you send:
- Guaranteed hard bounces. Every address at an empty-MX or tilde-MX domain will bounce. If your list was scraped or bought, a handful of these can add measurable percentage points to your hard-bounce rate in a single campaign.
- Slow-burn queue damage. Targets like
localhostand unresolvable names often fail as temporary errors first. Your infrastructure retries for days, spending connection slots and muddying your delivery metrics. - A signal about the whole record. A company whose MX is broken has not read email at that domain in a long time. Everything else you have on that account — the contact names, the titles — is probably equally stale.
The one-query pre-flight check
Before a list goes into a sequencer, resolve MX for every unique domain and drop the obvious corpses:
- No MX and no A record fallback — drop.
- Null MX (
0 .) — drop; the domain is telling you explicitly. - MX target empty,
localhost,~, a bare unqualified name, or anything in private IP space — drop. - MX resolves to a real, public hostname — keep, and classify the provider while you are there (see the pre-send MX guide).
Broken MX records are one corner of a much larger unknown: 12.45% of top-1M domains resolve to MX hosts no classifier recognises. The long-tail section of the daily report catalogues them, updated nightly.
MX validation removes the domains that cannot receive your mail. It says nothing about the harder question — whether the domains that can receive it will put it in the inbox. That part you verify the same way you verify everything in outbound: empirically, with a placement test against real mailboxes, before the campaign rather than after the damage.