{
 "metric": "mx-tls-cert",
 "segment": "gb-medium-large",
 "anchor": "company",
 "label": "UK medium & large companies",
 "population": "UK medium & large companies",
 "states": [
  "valid",
  "hostname_mismatch",
  "self_signed",
  "expired"
 ],
 "quiet_failure_states": [
  "self_signed",
  "expired"
 ],
 "measurement": "Complete STARTTLS, inspect the leaf certificate. Denominator: MXs offering STARTTLS.",
 "why_it_matters": "SMTP TLS is *opportunistic* (RFC 7435) — senders accept any certificate — so an invalid certificate still encrypts but authenticates nothing, leaving the connection open to interception. It only becomes a hard failure under DANE or MTA-STS `enforce`.",
 "limits": "Honesty demands the split. Because opportunistic TLS tolerates mismatches, only `self_signed` and `expired` are counted as the quiet failure; `hostname_mismatch` is reported but **not** counted.",
 "data_tier": "smtp",
 "population_predicate": "has_mx",
 "entity_type": "domain",
 "kind": "failure",
 "latest": {
  "day": "2026-08-14",
  "n": 419,
  "qf_pct": 1.91,
  "ci": [
   0.97,
   3.72
  ],
  "dist": {
   "valid": 93.08,
   "hostname_mismatch": 5.01,
   "self_signed": 1.43,
   "expired": 0.48
  },
  "snapshot": "uk-companies-house-per600-seed20260728"
 },
 "series": [
  {
   "day": "2026-08-02",
   "n": 409,
   "qf_pct": 1.71,
   "ci": [
    0.83,
    3.49
   ],
   "dist": {
    "self_signed": 0.73,
    "expired": 0.98,
    "valid": 94.87,
    "hostname_mismatch": 3.42
   },
   "snapshot": "uk-companies-house-per300-seed20260728"
  },
  {
   "day": "2026-08-04",
   "n": 422,
   "qf_pct": 1.66,
   "ci": [
    0.81,
    3.38
   ],
   "dist": {
    "valid": 93.6,
    "hostname_mismatch": 4.74,
    "self_signed": 0.95,
    "expired": 0.71
   },
   "snapshot": "uk-companies-house-per600-seed20260728"
  },
  {
   "day": "2026-08-05",
   "n": 423,
   "qf_pct": 1.89,
   "ci": [
    0.96,
    3.69
   ],
   "dist": {
    "valid": 93.14,
    "hostname_mismatch": 4.96,
    "self_signed": 1.42,
    "expired": 0.47
   },
   "snapshot": "uk-companies-house-per600-seed20260728"
  },
  {
   "day": "2026-08-07",
   "n": 417,
   "qf_pct": 1.44,
   "ci": [
    0.66,
    3.1
   ],
   "dist": {
    "valid": 94.24,
    "hostname_mismatch": 4.32,
    "self_signed": 0.96,
    "expired": 0.48
   },
   "snapshot": "uk-companies-house-per600-seed20260728"
  },
  {
   "day": "2026-08-14",
   "n": 419,
   "qf_pct": 1.91,
   "ci": [
    0.97,
    3.72
   ],
   "dist": {
    "valid": 93.08,
    "hostname_mismatch": 5.01,
    "self_signed": 1.43,
    "expired": 0.48
   },
   "snapshot": "uk-companies-house-per600-seed20260728"
  }
 ],
 "license": "CC BY 4.0",
 "source": "https://quietfailures.com/datasets/mx-tls-cert/gb-medium-large"
}