B2B Email Infrastructure Index 2026 Q3
Mail-receiving capability measured across 9,847,723 distinct domains. Released quarterly. Open data, reusable with attribution.
How many domains cannot receive mail at all?
815,487 of 9,847,723 measured domains - 8.28% - cannot receive mail by any route. The causes split three ways: 6.73% do not resolve, 1.15% publish a null MX under RFC 7505 to refuse mail explicitly, and 0.41% have neither an MX nor an address record.
Why do most published email-infrastructure figures overstate dead domains?
Because 552,369 domains publish no MX record but do publish an A or AAAA record. Under RFC 5321 section 5.1 those domains still receive mail by implicit MX. Counting them as dead inflates the dead-domain share by more than five percentage points.
What exactly was measured, and what was not?
One representative MX host per registered domain, resolved across the full set, with a second-pass address re-probe for domains returning nothing on the first pass. Prevalence is not deliverability: this measures which receiving stacks exist, never whether a given message will be accepted.
The data
| Metric | Domains | Share | Definition |
|---|---|---|---|
domains_measured | 9,847,723 | 100.0% | Distinct registered domains resolved for MX, one representative MX host per domain. |
mail_capable | 8,938,058 | 90.7627% | Can receive mail: publishes an MX record, or has no MX but a published A/AAAA record and therefore receives mail by implicit MX under RFC 5321 section 5.1. |
mail_capable_via_implicit_mx | 552,369 | 5.6091% | Subset of mail_capable: no MX record, but a published address record. Counting these as dead is the most common error in published email-infrastructure figures. |
cannot_receive_mail | 815,487 | 8.281% | Cannot receive mail by any route. Sum of the three rows below. |
nxdomain | 662,748 | 6.73% | Domain does not resolve at all. |
null_mx_rfc7505 | 112,822 | 1.1457% | Publishes a null MX under RFC 7505, explicitly refusing mail. |
no_mx_and_no_address | 39,917 | 0.4053% | No MX and no A/AAAA record, confirmed on re-probe. |
unknown | 93,605 | 0.9505% | Resolution inconclusive; reported rather than reassigned. |
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How should this be cited?
ReplyLead, B2B Email Infrastructure Index 2026 Q3, published 20 September 2026. Aggregate counts only; no domain names are published. Available at https://replylead.com/email-infrastructure-index-2026-q3.html under CC BY 4.0.