Mail transport encryption · UK small companies
What share of UK small companies have a mail server that accepts mail without offering STARTTLS?
2.15% of UK small companies (95% CI 1.13–4.03%), measured across 419 companies on 2026-10-01.
Measured every Friday since 2026-08-01 — 11 observations to date. Last measured 2026-10-01.
How this is measured
Connect to the lowest-preference MX on port 25, EHLO, check for the STARTTLS capability. Denominator: has_mx, further restricted to MXs that answer.
Passive measurement: a single DNS lookup, TLS handshake, or homepage GET per domain. Aggregate-only; no individual company is named. 95% Wilson confidence intervals. Population: national company registries (Companies House, Brreg, SIRENE), sampled by size band and resolved to email-active domains. Parked domains excluded. Snapshot: uk-companies-house-per600-seed20260728.
Licensed CC BY 4.0 — reuse with attribution to Quiet Failures. Download: JSON · CSV.
Why it matters
If STARTTLS is not offered, every inbound message crosses the internet in plaintext, readable by anything on the path. The domain may have perfect SPF/DMARC and still be handing its mail over in the clear.
Limits of this measurement
~32% of small-company MXs never answer within a 6s timeout and are excluded, so this is measured over reachable mail servers.
The same check, other segments
- France micro companies2.6%
- France large companies2.5%
- Norway large companies2.21%
- global domains2.03%
- UK companies1.89%
- France companies1.18%
- UK medium & large companies1.12%
- Norway micro companies0.81%