diff --git a/src/backend/controllers/auth/AuthController.ts b/src/backend/controllers/auth/AuthController.ts
index 25b07a932..091a5dbcf 100644
--- a/src/backend/controllers/auth/AuthController.ts
+++ b/src/backend/controllers/auth/AuthController.ts
@@ -22,7 +22,6 @@ import type { Request, RequestHandler, Response } from 'express';
 import crypto from 'node:crypto';
 import { posix as pathPosix } from 'node:path';
 import { v4 as uuidv4, validate as validateUuid } from 'uuid';
-import validator from 'validator';
 import { Controller, Get, Post } from '../../core/http/decorators.js';
 import type { HttpErrorOptions } from '../../core/http/HttpError.js';
 import { HttpError } from '../../core/http/HttpError.js';
@@ -77,7 +76,11 @@ import {
     phoneAttemptsKey,
 } from '../../util/cardFallback.js';
 import { sessionCookieFlags } from '../../util/cookieFlags.js';
-import { cleanEmail, isBlockedEmail } from '../../util/email.js';
+import {
+    cleanEmail,
+    isBlockedEmail,
+    isStorableEmail,
+} from '../../util/email.js';
 import { isGodmodeApp } from '../../util/godmodeApps.js';
 import { generate_identifier } from '../../util/identifier.js';
 import { parsePhone } from '../../util/phone.js';
@@ -802,7 +805,7 @@ export class AuthController extends PuterController {
                 throw new HttpError(400, 'email must be a string.', {
                     legacyCode: 'bad_request',
                 });
-            if (!validator.isEmail(body.email))
+            if (!isStorableEmail(body.email))
                 throw new HttpError(
                     400,
                     'Please enter a valid email address.',
@@ -2414,7 +2417,7 @@ export class AuthController extends PuterController {
         if (username) {
             user = await this.stores.user.getByUsername(username);
         } else {
-            if (!validator.isEmail(email))
+            if (!isStorableEmail(email))
                 throw new HttpError(400, 'Invalid email.', {
                     legacyCode: 'bad_request',
                 });
@@ -2808,7 +2811,7 @@ export class AuthController extends PuterController {
                 legacyCode: 'bad_request',
             });
         }
-        if (!validator.isEmail(new_email)) {
+        if (!isStorableEmail(new_email)) {
             throw new HttpError(400, 'Please enter a valid email address.', {
                 legacyCode: 'bad_request',
             });
@@ -3053,7 +3056,7 @@ export class AuthController extends PuterController {
                 legacyCode: 'username_already_in_use',
             });
         }
-        if (!email || !validator.isEmail(email)) {
+        if (!email || !isStorableEmail(email)) {
             throw new HttpError(400, 'Please enter a valid email address.', {
                 legacyCode: 'bad_request',
             });
diff --git a/src/backend/services/share/ShareService.ts b/src/backend/services/share/ShareService.ts
index feed605a3..7a0e33900 100644
--- a/src/backend/services/share/ShareService.ts
+++ b/src/backend/services/share/ShareService.ts
@@ -280,7 +280,8 @@ export const DEFAULT_DAILY_SHARE_LIMIT = 200;
  * Deliverability is the inbox's business, but `a@b` or a pasted sentence must
  * not become a permanent pending share that spent quota.
  */
-const EMAIL_SHAPE = /^[^\s@]+@[^\s@]+\.[^\s@]+$/u;
+// `"<>` excluded too: legal only in a quoted local part nothing uses.
+const EMAIL_SHAPE = /^[^\s@"<>]+@[^\s@"<>]+\.[^\s@"<>]+$/u;
 
 /**
  * Where "refuse shares from everyone" lives on the user row.
diff --git a/src/backend/services/team/TeamProvisioning.test.ts b/src/backend/services/team/TeamProvisioning.test.ts
index 196b6b2fc..aa08a7250 100644
--- a/src/backend/services/team/TeamProvisioning.test.ts
+++ b/src/backend/services/team/TeamProvisioning.test.ts
@@ -95,6 +95,25 @@ describe('team account provisioning: home path conflicts', () => {
         ).resolves.toBeNull();
     });
 
+    it('refuses a seat email that could render as markup', async () => {
+        const owner = await makeUser();
+        const team = await makeTeam(owner.id);
+        const username = `tp_${Math.random().toString(36).slice(2, 10)}`;
+
+        // A quoted local part passes `validator.isEmail`; it must not store.
+        await expect(
+            service.provisionAccount(team.uid, owner.id, {
+                username,
+                email: '"<img/src=x/onerror=alert(1)>"@example.com',
+            }),
+        ).rejects.toMatchObject({ statusCode: 400 });
+
+        // Refused before any write, so the name is still free.
+        await expect(
+            server.stores.user.getByUsername(username, { force: true }),
+        ).resolves.toBeNull();
+    });
+
     it('does not suggest a name whose home path is occupied', async () => {
         const owner = await makeUser();
         await makeTeam(owner.id);
diff --git a/src/backend/services/team/TeamService.ts b/src/backend/services/team/TeamService.ts
index 9c7160fc0..3bcad37bb 100644
--- a/src/backend/services/team/TeamService.ts
+++ b/src/backend/services/team/TeamService.ts
@@ -18,7 +18,6 @@
  */
 
 import bcrypt from 'bcrypt';
-import validator from 'validator';
 import { v4 as uuidv4 } from 'uuid';
 import {
     USERNAME_MAX_LENGTH,
@@ -56,7 +55,7 @@ import {
     type PageResult,
 } from '../../util/pagination.js';
 import type { UserRow } from '../../stores/user/UserStore';
-import { cleanEmail } from '../../util/email.js';
+import { cleanEmail, isStorableEmail } from '../../util/email.js';
 import {
     generateTemporaryPassword,
     temporaryPasswordExpiry,
@@ -820,18 +819,16 @@ export class TeamService extends PuterService {
             id === null ? null : (users.get(id)?.username ?? null);
 
         return {
-            items: page.items.map(
-                (row): MemberActivityEntry => ({
-                    action: row.action,
-                    reason: row.reason,
-                    created_at: epochSeconds(row.created_at),
-                    username: name(row.user_id_keep),
-                    actor_username: name(row.actor_user_id),
-                    // Only a sign-in carries these; the shape stays uniform.
-                    ip: null,
-                    user_agent: null,
-                }),
-            ),
+            items: page.items.map((row): MemberActivityEntry => ({
+                action: row.action,
+                reason: row.reason,
+                created_at: epochSeconds(row.created_at),
+                username: name(row.user_id_keep),
+                actor_username: name(row.actor_user_id),
+                // Only a sign-in carries these; the shape stays uniform.
+                ip: null,
+                user_agent: null,
+            })),
             ...(page.cursor ? { cursor: page.cursor } : {}),
         };
     }
@@ -917,7 +914,7 @@ export class TeamService extends PuterService {
 
         this.#assertUsableUsername(input.username);
         const email = typeof input.email === 'string' ? input.email.trim() : '';
-        if (email && !validator.isEmail(email)) {
+        if (email && !isStorableEmail(email)) {
             throw new HttpError(400, 'Invalid email', {
                 legacyCode: 'bad_request',
             });
diff --git a/src/backend/util/email.test.ts b/src/backend/util/email.test.ts
index c397cc48d..7b7882281 100644
--- a/src/backend/util/email.test.ts
+++ b/src/backend/util/email.test.ts
@@ -18,7 +18,8 @@
  */
 
 import { describe, expect, it } from 'vitest';
-import { cleanEmail, isBlockedEmail } from './email.ts';
+import validator from 'validator';
+import { cleanEmail, isBlockedEmail, isStorableEmail } from './email.ts';
 
 describe('cleanEmail', () => {
     it('lowercases the whole address', () => {
@@ -94,3 +95,28 @@ describe('isBlockedEmail', () => {
         ).toBe(true);
     });
 });
+
+describe('isStorableEmail', () => {
+    const markup = '"<img/src=x/onerror=alert(1)>"@example.com';
+
+    it('refuses a quoted local part that carries markup', () => {
+        // The reason this gate exists: the library on its own says yes.
+        expect(validator.isEmail(markup)).toBe(true);
+        expect(isStorableEmail(markup)).toBe(false);
+    });
+
+    it('refuses the bare metacharacters too', () => {
+        for (const bad of ['a<b@example.com', 'a>b@example.com', 'a"b@example.com'])
+            expect(isStorableEmail(bad)).toBe(false);
+    });
+
+    it('still accepts addresses people actually have', () => {
+        for (const good of [
+            "o'brien@example.com",
+            'a&b@example.com',
+            'foo.bar+tag@gmail.com',
+            'user_name-1@sub.example.co.uk',
+        ])
+            expect(isStorableEmail(good)).toBe(true);
+    });
+});
diff --git a/src/backend/util/email.ts b/src/backend/util/email.ts
index 201fd9af7..aeeb41237 100644
--- a/src/backend/util/email.ts
+++ b/src/backend/util/email.ts
@@ -25,6 +25,8 @@
  * isBlockedEmail('temp@mailinator.com', ['mailinator.com']) === true
  */
 
+import validator from 'validator';
+
 type RuleName = 'dots_dont_matter' | 'remove_subaddressing';
 
 interface Parts {
@@ -103,6 +105,11 @@ export function cleanEmail(email: string): string {
     return `${parts.local}@${parts.domain}`;
 }
 
+/** Storage gate; `isEmail` alone passes quoted local parts carrying markup. */
+export function isStorableEmail(email: string): boolean {
+    return !/["<>]/u.test(email) && validator.isEmail(email);
+}
+
 /** Strips `+` on any domain. For abuse decisions only, never for identity. */
 export function abuseKey(email: string): string {
     const [local, domain] = cleanEmail(email).split('@');