diff --git a/.agents/skills/redacted-circuit-maintenance/SKILL.md b/.agents/skills/redacted-circuit-maintenance/SKILL.md
index 71c8624..4c556d2 100644
--- a/.agents/skills/redacted-circuit-maintenance/SKILL.md
+++ b/.agents/skills/redacted-circuit-maintenance/SKILL.md
@@ -83,7 +83,7 @@ Each section below is a rule plus the incident that produced it (2026-09-27, PR
 - Keys come from `src/utils/dkimKeys.ts` (a copy lives in the CLI). It tries Google + Cloudflare
   DoH first, then every archived key for the selector. `@zk-email/helpers` alone tries one DNS
   key, and only the first archived key, and only if the DNS lookup fails. That missed 12 of 132
-  corpus emails whose keys were rotated under the same selector (mostly Amazon/SES).
+  corpus emails whose keys were rotated under the same selector.
 - archive.prove.email allows **10 requests/min per IP**. On a 429 it returns an error object, and
   the helper's own fallback crashes ("data.find is not a function"). An unpaced corpus
   DKIM sweep therefore misreports ~30 emails as failures. When sweeping, space archive calls
diff --git a/skills/redacted-email-proof/SKILL.md b/skills/redacted-email-proof/SKILL.md
index 25ce412..5bc9dc0 100644
--- a/skills/redacted-email-proof/SKILL.md
+++ b/skills/redacted-email-proof/SKILL.md
@@ -57,7 +57,7 @@ the smallest tier that fits (RSA 1024- or 2048-bit keys):
 | large | 4,096 B | 48,000 B | ~1 min on 18 cores (longer on fewer), **~7 GB RAM** |
 | xl | 4,096 B | 100,352 B | ~2–4 min on 18 cores (longer on fewer), **~14 GB RAM** (16 GB+ machine) |
 
-The large-tier figures were measured on a 34 KB PayPal receipt: 58 s, 6.6 GB peak for the native
+The large-tier figures were measured on a real 34 KB receipt: 58 s, 6.6 GB peak for the native
 `bb`. XL figures were measured on 87.6 KB and 94.9 KB receipts (loaded machine; faster idle). Peak
 memory is the kernel's lifetime physical footprint of the `bb` process (`/usr/bin/time -l`): large
 7.4 GB, XL 13.7 GB (18-core M-series Mac, 2026-09-29).
diff --git a/skills/redacted-email-proof/scripts/redacted.mjs b/skills/redacted-email-proof/scripts/redacted.mjs
index 4481106..f8f3993 100755
--- a/skills/redacted-email-proof/scripts/redacted.mjs
+++ b/skills/redacted-email-proof/scripts/redacted.mjs
@@ -737,6 +737,34 @@ export function domainsAligned(fromDomain, signingDomain) {
   return f === d || f.endsWith("." + d) || d.endsWith("." + f);
 }
 
+// REASON: Google Workspace signs mail for domains without their own DKIM as
+// d=<from-domain, dots as dashes>.<yyyymmdd>.gappssmtp.com, and Microsoft 365 as
+// d=<tenant>.onmicrosoft.com. A Google label equal to the dashed From: domain ties the signature
+// to that domain's Workspace (Google signs only for its customers and only From: addresses they
+// verified); Microsoft tenant names aren't tied to domains, so those stay unproven.
+// NOTE: dots->dashes is ambiguous (bank.co.uk / bank-co.uk -> bank-co-uk) and Workspace's SMTP
+// relay can send any From:, so only one-dot From: domains (unambiguous) count as matched.
+// Same as src/utils/keyBinding.ts signerProvider (see its NOTE on the dots->dashes collision).
+export function signerProvider(fromDomain, signingDomain) {
+  const d = signingDomain.toLowerCase();
+  const g = /^([a-z0-9-]+)\.\d{8}\.gappssmtp\.com$/.exec(d);
+  if (g) {
+    const f = fromDomain?.toLowerCase() ?? "";
+    const unambiguous = f.split(".").length === 2;
+    return { kind: "google-workspace", tenant: g[1], matchesFrom: unambiguous && g[1] === f.replace(/\./g, "-") };
+  }
+  const ms = /^([a-z0-9-]+)\.onmicrosoft\.com$/.exec(d);
+  if (ms) return { kind: "microsoft-365", tenant: ms[1], matchesFrom: false };
+  return null;
+}
+
+function alignmentOf(fromDomain, signingDomain) {
+  const provider = signerProvider(fromDomain, signingDomain);
+  if (fromDomain && domainsAligned(fromDomain, signingDomain)) return { fromAligned: true, alignedVia: "domain", provider };
+  if (provider?.matchesFrom) return { fromAligned: true, alignedVia: "google-workspace", provider };
+  return { fromAligned: false, alignedVia: null, provider };
+}
+
 // Decode the masked header/body a proof publishes (0x00 = masked). v2 (prefix 5) commits to the
 // signed lengths and zeroes everything past them, so this is an exact slice.
 // Throws on non-canonical inputs (see canonicalPublicInputs).
@@ -924,7 +952,7 @@ export async function checkKeyBinding(publicInputs, header, circuit, resolveKeys
         if (!n) continue;
         const got = await pubkeyHash(n, keyBits, version);
         if (got.length === want.length && got.every((v, i) => v === want[i])) {
-          return { ...out, domain: d, selector: s, matched: out.tried.at(-1), fromAligned: from.domain ? domainsAligned(from.domain, d) : false };
+          return { ...out, domain: d, selector: s, matched: out.tried.at(-1), ...alignmentOf(from.domain, d) };
         }
       }
     }
@@ -1173,6 +1201,8 @@ async function cmdVerify(ref, opts) {
     fromAddress: binding?.from?.address ?? null,
     fromHidden: binding?.from?.hidden ?? false,
     fromAlignedWithDkimDomain: binding?.fromAligned ?? null,
+    fromAlignedVia: binding?.alignedVia ?? null,
+    signerProvider: binding?.provider ?? null,
     maskedHeader: outs.headerText,
     maskedBody: outs.bodyText,
   };
@@ -1195,6 +1225,12 @@ async function cmdVerify(ref, opts) {
             : `dkim key: NOT matched to any published key for ${pairs} ` +
               `(tried: ${result.keysTried.join("; ") || "no key found"}${binding.archiveError ? `; archive: ${binding.archiveError}` : ""}). Treat the sender as unproven.`,
       );
+      if (result.keyMatches && result.fromAlignedVia === "google-workspace") {
+        console.log(
+          `from: ${result.fromAddress} — signed by Google Workspace for ${binding.from.domain} (d=${result.dkimDomain}; ` +
+            `Google signs this way for Workspace domains without their own DKIM key).`,
+        );
+      }
       if (result.keyMatches && result.fromAlignedWithDkimDomain === false) {
         // REASON: usually innocent (a mailing service signing with its own domain), so name the
         // signing domain and the matching key, and say that only the From: line is unproven.
@@ -1203,8 +1239,11 @@ async function cmdVerify(ref, opts) {
           result.fromHidden
             ? `warning: the ${signer}, but the From: address is hidden, so it isn't proven to be on ${result.dkimDomain}.`
             : result.fromAddress
-              ? `warning: From: ${result.fromAddress} is on ${binding.from.domain}, but the ${signer}. This is often a quirk ` +
-                `of the sender's email service (it signs with its own domain), but the From: address is NOT proven.`
+              ? `warning: From: ${result.fromAddress} is on ${binding.from.domain}, but the ${signer}. ` +
+                (result.signerProvider?.kind === "microsoft-365"
+                  ? `${result.dkimDomain} is a Microsoft 365 tenant (Microsoft signs this way when a domain has no DKIM key of its own), ` +
+                    `and which domains a tenant owns isn't public, so the From: address is NOT proven.`
+                  : `This is often a quirk of the sender's email service (it signs with its own domain), but the From: address is NOT proven.`)
               : `warning: the ${signer}, but the From: address can't be read, so it isn't proven.`,
         );
       }
diff --git a/src/utils/keyBinding.ts b/src/utils/keyBinding.ts
index c9e923e..73f8483 100644
--- a/src/utils/keyBinding.ts
+++ b/src/utils/keyBinding.ts
@@ -120,6 +120,46 @@ export function domainsAligned(fromDomain: string, signingDomain: string): boole
   return f === d || f.endsWith("." + d) || d.endsWith("." + f);
 }
 
+export interface SignerProvider {
+  kind: "google-workspace" | "microsoft-365";
+  /** the tenant label in d= */
+  tenant: string;
+  /** the tenant provably belongs to the From: domain */
+  matchesFrom: boolean;
+}
+
+/**
+ * Mail platforms that sign customer mail with their own domain when the customer hasn't set up
+ * DKIM for its domain.
+ * REASON: plain domain alignment calls these "From: not proven", although for Google Workspace the
+ * d= names the customer's domain:
+ *  - Google Workspace signs as d=<from-domain, dots as dashes>.<yyyymmdd>.gappssmtp.com
+ *    (school.example -> school-example.20230601.gappssmtp.com). Google signs that way only for its own
+ *    Workspace customers, so a label equal to the dashed From: domain ties the signature to that
+ *    domain's Workspace.
+ *    NOTE: dots->dashes is not one-to-one: bank.co.uk and bank-co.uk both give bank-co-uk, and
+ *    mail.bank.example / mail-bank.example both give mail-bank-example. Workspace's SMTP relay can be
+ *    set to send any From: address, so the owner of a colliding domain could produce this exact d=.
+ *    The mapping is only unambiguous when the From: domain has exactly one dot (a TLD can't contain
+ *    a plain hyphen), so matchesFrom is limited to those. Longer domains stay unproven with a
+ *    Google-specific explanation. (Same reasoning made zk-email-verify#324 keep gappssmtp opt-in.)
+ *  - Microsoft 365 signs as d=<tenant>.onmicrosoft.com. The tenant name isn't derived from any
+ *    domain and which domains a tenant owns isn't public, so the From: line stays unproven. The
+ *    banner just explains the quirk.
+ */
+export function signerProvider(fromDomain: string | null, signingDomain: string): SignerProvider | null {
+  const d = signingDomain.toLowerCase();
+  const g = /^([a-z0-9-]+)\.\d{8}\.gappssmtp\.com$/.exec(d);
+  if (g) {
+    const f = fromDomain?.toLowerCase() ?? "";
+    const unambiguous = f.split(".").length === 2;
+    return { kind: "google-workspace", tenant: g[1], matchesFrom: unambiguous && g[1] === f.replace(/\./g, "-") };
+  }
+  const ms = /^([a-z0-9-]+)\.onmicrosoft\.com$/.exec(d);
+  if (ms) return { kind: "microsoft-365", tenant: ms[1], matchesFrom: false };
+  return null;
+}
+
 // ---------------------------------------------------------------------------------------------
 // key binding
 
@@ -194,6 +234,10 @@ export interface KeyBinding {
   from: ReturnType<typeof fromAddress>;
   /** From: domain aligned with the matched signing domain; null when there is no match */
   fromAligned: boolean | null;
+  /** how fromAligned was established */
+  alignedVia?: "domain" | "google-workspace";
+  /** the signer is a mail platform's domain (Google Workspace, Microsoft 365) */
+  provider?: SignerProvider | null;
   /** archive lookup failed (e.g. its 10 req/min limit); an unmatched result may be a false negative */
   archiveError?: string;
 }
@@ -237,7 +281,7 @@ export async function bindDkimKey(
             domain,
             selector,
             keySource: k.source,
-            fromAligned: from.domain ? domainsAligned(from.domain, domain) : false,
+            ...alignmentOf(from.domain, domain),
           };
         }
       }
@@ -246,6 +290,13 @@ export async function bindDkimKey(
   return result;
 }
 
+function alignmentOf(fromDomain: string | null, signingDomain: string) {
+  const provider = signerProvider(fromDomain, signingDomain);
+  if (fromDomain && domainsAligned(fromDomain, signingDomain)) return { fromAligned: true, alignedVia: "domain" as const, provider };
+  if (provider?.matchesFrom) return { fromAligned: true, alignedVia: "google-workspace" as const, provider };
+  return { fromAligned: false, provider };
+}
+
 /** Banner for a verified v2 proof, from its DKIM key binding. Used by the verify page; tested in tests/key-binding.test.ts. */
 export function keyBindingStatus(b: KeyBinding): { verified: boolean; warning?: boolean; message: string } {
   if (!b.matched) {
@@ -262,6 +313,15 @@ export function keyBindingStatus(b: KeyBinding): { verified: boolean; warning?:
   }
   const keyFrom = b.keySource === "archive" ? "the DKIM key archive (no longer in DNS)" : "DNS";
   const key = `the DKIM key published at ${b.selector}._domainkey.${b.domain} (${keyFrom})`;
+  if (b.fromAligned && b.alignedVia === "google-workspace") {
+    return {
+      verified: true,
+      message:
+        `Proof verified. Signed by Google Workspace for ${b.from.domain}: the proof's key matches ${key}. ` +
+        `Google signs as ${b.domain} for Workspace domains that haven't set up their own DKIM key, and the ` +
+        `signature names ${b.from.domain}'s Workspace.`,
+    };
+  }
   if (b.fromAligned) {
     return { verified: true, message: `Proof verified. Signed by ${b.domain}: the proof's key matches ${key}, and the From: address is on that domain.` };
   }
@@ -269,8 +329,17 @@ export function keyBindingStatus(b: KeyBinding): { verified: boolean; warning?:
   // school/church/HR systems) send on a company's behalf and sign with THEIR domain. Say exactly
   // which domain signed and which key matched, and that the From: line itself isn't proven.
   const quirk =
-    "This is often just how the sender's email service works: services that send mail on someone's behalf " +
-    "commonly sign with their own domain.";
+    b.provider?.kind === "microsoft-365"
+      ? `${b.domain} is a Microsoft 365 tenant: Microsoft signs this way when a domain hasn't set up its own DKIM key. ` +
+        "Which domains a tenant owns isn't public, so this can't be tied to the From: domain."
+      : b.provider?.kind === "google-workspace"
+        ? b.from.domain && b.provider.tenant === b.from.domain.replace(/\./g, "-")
+          ? `${b.domain} is Google Workspace signing for "${b.provider.tenant}". That name matches ${b.from.domain}, but ` +
+            `Google writes dots as dashes, so it would also match other domains (e.g. ${b.from.domain.replace(/\.(?=[^.]*\.)/, "-")}); ` +
+            "the From: domain isn't proven."
+          : `${b.domain} is Google Workspace signing for the Workspace "${b.provider.tenant}", which doesn't match the From: domain.`
+        : "This is often just how the sender's email service works: services that send mail on someone's behalf " +
+          "commonly sign with their own domain.";
   return {
     verified: false,
     warning: true,
diff --git a/tests/key-binding.test.ts b/tests/key-binding.test.ts
index f0a168e..0d189a2 100644
--- a/tests/key-binding.test.ts
+++ b/tests/key-binding.test.ts
@@ -275,3 +275,55 @@ test("real proofs: non-canonical inputs, key match, forged key, unaligned From (
     await api.destroy();
   }
 });
+
+// Google Workspace / Microsoft 365 sign customer mail with their own domain when the customer has
+// no DKIM key of its own (see signerProvider). Shape: From school.example, d=school-example.20230601.gappssmtp.com.
+test("signerProvider: Google Workspace tenant label = dashed From: domain; Microsoft 365 never ties", () => {
+  for (const [name, m] of both) {
+    assert.deepEqual(m.signerProvider("school.example", "school-example.20230601.gappssmtp.com"),
+      { kind: "google-workspace", tenant: "school-example", matchesFrom: true }, name);
+    assert.equal(m.signerProvider("bank.example", "school-example.20230601.gappssmtp.com").matchesFrom, false, name);
+    assert.equal(m.signerProvider(null, "school-example.20230601.gappssmtp.com").matchesFrom, false, name);
+    assert.equal(m.signerProvider("my-site.example", "my-site-example.20150623.gappssmtp.com").matchesFrom, true, name);
+    // ambiguous: bank.co.uk and bank-co.uk both map to bank-co-uk, so neither may claim it
+    assert.equal(m.signerProvider("bank.co.uk", "bank-co-uk.20230601.gappssmtp.com").matchesFrom, false, name);
+    assert.equal(m.signerProvider("mail.bank.example", "mail-bank-example.20230601.gappssmtp.com").matchesFrom, false, name);
+    assert.deepEqual(m.signerProvider("school.example", "school.onmicrosoft.com"),
+      { kind: "microsoft-365", tenant: "school", matchesFrom: false }, name);
+    assert.equal(m.signerProvider("bank.example", "esp.example"), null, name);
+    // not Google's shape: no 8-digit date label, or another parent domain
+    assert.equal(m.signerProvider("school.example", "school-example.gappssmtp.com"), null, name);
+    assert.equal(m.signerProvider("school.example", "school-example.20230601.gappssmtp.com.evil.example"), null, name);
+  }
+});
+
+const workspaceBinding = (fromDomain: string, d: string) => {
+  const p = site.signerProvider(fromDomain, d);
+  const aligned = site.domainsAligned(fromDomain, d) || !!p?.matchesFrom;
+  return {
+    matched: true, domain: d, selector: "20230601", keySource: "dns:google",
+    candidates: [{ domain: d, selector: "20230601" }],
+    from: { address: `principal@${fromDomain}`, domain: fromDomain, hidden: false },
+    fromAligned: aligned, alignedVia: aligned ? ("google-workspace" as const) : undefined, provider: p,
+  };
+};
+
+test("verify banner: Google Workspace signature for the From: domain is verified and says so", () => {
+  const s = site.keyBindingStatus(workspaceBinding("school.example", "school-example.20230601.gappssmtp.com"));
+  assert.equal(s.verified, true);
+  assert.match(s.message, /Google Workspace for school\.example/);
+  const other = site.keyBindingStatus(workspaceBinding("bank.example", "school-example.20230601.gappssmtp.com"));
+  assert.equal(other.verified, false);
+  assert.equal(other.warning, true);
+  assert.match(other.message, /doesn't match the From: domain/);
+  const ambiguous = site.keyBindingStatus(workspaceBinding("bank.co.uk", "bank-co-uk.20230601.gappssmtp.com"));
+  assert.equal(ambiguous.verified, false);
+  assert.match(ambiguous.message, /would also match other domains/);
+});
+
+test("verify banner: Microsoft 365 tenant stays unproven, with a specific explanation", () => {
+  const s = site.keyBindingStatus(workspaceBinding("school.example", "school.onmicrosoft.com"));
+  assert.equal(s.verified, false);
+  assert.equal(s.warning, true);
+  assert.match(s.message, /Microsoft 365 tenant/);
+});