diff --git a/.changeset/27503-mention-email-case-insensitive.md b/.changeset/27503-mention-email-case-insensitive.md
new file mode 100644
index 00000000000..80313cb246d
--- /dev/null
+++ b/.changeset/27503-mention-email-case-insensitive.md
@@ -0,0 +1,10 @@
+---
+'@objectstack/plugin-audit': patch
+---
+
+fix(plugin-audit): an @mention by email matches the user whatever the address's casing
+
+Clause-②: no
+Metadata-protocol: none
+
+A comment that @-mentioned a colleague by email in a different case than the stored address (`Carol@Example.com` for `carol@example.com`) was accepted, but the mention resolved no user and the colleague was never notified; the server logged `mention notification withheld` with reason `unknown_email`. The mention lookup now normalises the address the way identity and sign-in do (trim, then lowercase, through the shared `normalizePlatformAdminEmail`), so it names the same user sign-in would. Who may be notified is unchanged: the record and comment read checks still decide.
diff --git a/packages/plugins/plugin-audit/src/mention-recipient-gate.integration.test.ts b/packages/plugins/plugin-audit/src/mention-recipient-gate.integration.test.ts
index 7089c410001..d992a8476f1 100644
--- a/packages/plugins/plugin-audit/src/mention-recipient-gate.integration.test.ts
+++ b/packages/plugins/plugin-audit/src/mention-recipient-gate.integration.test.ts
@@ -279,6 +279,13 @@ describe('@mention notifications reach only mentioned users who can read the com
       expect(audiences(await comment(openTask, [CAROL.email]))).toEqual([[CAROL.id]]);
     });
 
+    it('resolves an email mention whatever its casing, the way sign-in finds the user', async () => {
+      // The stored address is the lowercased one identity writes; sign-in
+      // looks it up lowercased, and so must a mention.
+      expect(audiences(await comment(openTask, ['Carol@A.Test']))).toEqual([[CAROL.id]]);
+      expect(audiences(await comment(openTask, [` ${CAROL.email.toUpperCase()} `]))).toEqual([[CAROL.id]]);
+    });
+
     it('never notifies the author, by id or by email', async () => {
       expect(audiences(await comment(openTask, [ALICE.id, ALICE.email]))).toEqual([]);
     });
@@ -367,6 +374,20 @@ describe('@mention notifications reach only mentioned users who can read the com
       expect(JSON.stringify(warned)).not.toContain(String(row.body));
     });
 
+    it('logs no unknown_email for a differently-cased address of a stored user', async () => {
+      const commentId = String((await post(openTask, [])).id);
+      const warned: Array<{ message: string; meta?: Record<string, unknown> }> = [];
+      const admitted = await admitMentionRecipients(gate(warned), {
+        mentions: ['CAROL@A.TEST'],
+        threadId: `${TASK}:${openTask}`,
+        actorId: ALICE.id,
+        organizationId: ORG_A,
+        commentId,
+      });
+      expect(admitted).toEqual([CAROL.id]);
+      expect(warned).toEqual([]);
+    });
+
     it('withholds every recipient when the comment cannot be read back by its id', async () => {
       const warned: Array<{ message: string; meta?: Record<string, unknown> }> = [];
       const input = { mentions: [CAROL.id], threadId: `${TASK}:${openTask}`, actorId: ALICE.id, organizationId: ORG_A };
diff --git a/packages/plugins/plugin-audit/src/mention-recipient-gate.ts b/packages/plugins/plugin-audit/src/mention-recipient-gate.ts
index 1ee01f81f18..8f07337134c 100644
--- a/packages/plugins/plugin-audit/src/mention-recipient-gate.ts
+++ b/packages/plugins/plugin-audit/src/mention-recipient-gate.ts
@@ -54,6 +54,7 @@
  */
 
 import {
+  normalizePlatformAdminEmail,
   resolveReadableParentIds,
   resolveUserAuthzGrants,
   vetOrganizationClaim,
@@ -254,6 +255,13 @@ type MentionedUser = { ok: true; id: string } | { ok: false; reason: MentionWith
  * user may be notified is the read check's question, never this lookup's).
  * An audience expansion (`role:` / `team:` / `owner_of:`) names no single
  * person and is not a mention.
+ *
+ * An email is looked up the way sign-in looks one up: identity stores
+ * `sys_user.email` lowercased, and sign-in queries the lowercased address, so
+ * a mention normalises with the platform's one email normalisation
+ * (`normalizePlatformAdminEmail`: trim, then lowercase) before the exact
+ * match. ⛔ No second email matcher here — a differently-cased address must
+ * name the same user sign-in would.
  */
 async function resolveMentionedUser(engine: MentionRecipientGateEngine, raw: string): Promise<MentionedUser> {
   let value = raw.trim();
@@ -263,7 +271,7 @@ async function resolveMentionedUser(engine: MentionRecipientGateEngine, raw: str
   if (!value.includes('@')) return { ok: true, id: value };
   try {
     const rows = await engine.find(USER_OBJECT, {
-      where: { email: value },
+      where: { email: normalizePlatformAdminEmail(value) },
       fields: ['id'],
       limit: 1,
       context: SYSTEM_CONTEXT,