diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index 74561b685..5a28842ed 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -56,7 +56,7 @@ import { debug } from "@/lib/debug";
 import { playNotificationSound } from "@/lib/notification-sound";
 import { cn, getMailboxPath } from "@/lib/utils";
 import { localizeMailboxName } from "@/lib/mailbox-label";
-import { KEYWORD_PREFIX, KEYWORD_PREFIX_LEGACY, groupEmailsByThread, threadKeyFor } from "@/lib/thread-utils";
+import { groupEmailsByThread, threadKeyFor, toggleTagKeywords } from "@/lib/thread-utils";
 import { resolveThreadRoute } from "@/lib/thread-routing";
 import {
   ErrorBoundary,
@@ -1589,19 +1589,25 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
     const markAsReadDelay = useSettingsStore.getState().markAsReadDelay;
     debug.log('email', '[Mark as Read] Delay setting:', markAsReadDelay, 'ms for email:', selectedEmail.id);
 
+    // The server may refuse (a shared folder without the right to set seen);
+    // the message then just stays unread.
+    const markRead = () => markAsRead(client, selectedEmail.id, true).catch((error) => {
+      debug.warn('email', '[Mark as Read] Failed:', error);
+    });
+
     if (markAsReadDelay === -1) {
       // Never mark as read automatically
       debug.log('email', '[Mark as Read] Never mode - email will stay unread');
     } else if (markAsReadDelay === 0) {
       // Mark as read instantly
       debug.log('email', '[Mark as Read] Instant mode - marking as read now');
-      markAsRead(client, selectedEmail.id, true);
+      void markRead();
     } else {
       // Mark as read after delay
       debug.log('email', '[Mark as Read] Delayed mode - will mark as read in', markAsReadDelay, 'ms');
       markAsReadTimeoutRef.current = setTimeout(() => {
         debug.log('email', '[Mark as Read] Timeout fired - marking as read now');
-        markAsRead(client, selectedEmail.id, true);
+        void markRead();
         markAsReadTimeoutRef.current = null;
       }, markAsReadDelay);
     }
@@ -2426,33 +2432,12 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
     if (!client) return;
 
     try {
-      // Remove any existing tag keywords
       const email = emails.find(e => e.id === emailId);
       if (!email) return;
 
-      const keywords = { ...email.keywords };
-
-      if (tagId === null) {
-        // Remove all tag keywords
-        Object.keys(keywords).forEach(key => {
-          if (key.startsWith(KEYWORD_PREFIX) || key.startsWith(KEYWORD_PREFIX_LEGACY)) {
-            keywords[key] = false;
-          }
-        });
-      } else {
-        // Both prefixes name the same tag when read, so taking one off has to
-        // clear whichever spellings are actually set.
-        const activeKeys = [KEYWORD_PREFIX + tagId, KEYWORD_PREFIX_LEGACY + tagId]
-          .filter(key => keywords[key]);
-        if (activeKeys.length > 0) {
-          activeKeys.forEach(key => {
-            keywords[key] = false;
-          });
-        } else {
-          // Add the tag without disturbing others
-          keywords[KEYWORD_PREFIX + tagId] = true;
-        }
-      }
+      // The whole keyword map is written, so it may only hold keywords that
+      // are set: a removed tag is left out rather than sent as false.
+      const keywords = toggleTagKeywords(email.keywords, tagId);
 
       // Routes the write to the email's own account, then patches the email in
       // place so the list keeps its scroll/pagination state instead of being
@@ -2465,7 +2450,10 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
       // Refresh tag counts
       fetchTagCounts(client);
     } catch (error) {
+      // The message keeps its old tags. Stalwart 1.0 refuses keywords over
+      // 128 bytes and more than 100 on one message.
       console.error("Failed to set tag:", error);
+      toast.error(t('notifications.tag_failed'));
     }
   };
 
diff --git a/components/settings/__tests__/keyword-settings.test.tsx b/components/settings/__tests__/keyword-settings.test.tsx
index 9a24d114f..12bc04bdd 100644
--- a/components/settings/__tests__/keyword-settings.test.tsx
+++ b/components/settings/__tests__/keyword-settings.test.tsx
@@ -163,8 +163,10 @@ describe('KeywordSettings', () => {
     expect(screen.getByText('$label:work/clients')).toBeInTheDocument();
   });
 
-  it('rejects a path that would exceed the keyword length limit', () => {
-    const deepId = 'a'.repeat(240);
+  it('rejects a path that would exceed the 128-byte keyword limit of Stalwart', () => {
+    // `$label:` + 110 + `/overflowing-name` is 134 bytes: fine by the RFC's
+    // 255, refused by Stalwart 1.0 and cut short by 0.16.
+    const deepId = 'a'.repeat(110);
     useSettingsStore.setState({
       emailKeywords: [{ id: deepId, label: 'Deep', color: 'blue' }],
       nestedTags: true,
diff --git a/lib/__tests__/jmap-cross-account-move.test.ts b/lib/__tests__/jmap-cross-account-move.test.ts
index 462abe5b8..b310bf39a 100644
--- a/lib/__tests__/jmap-cross-account-move.test.ts
+++ b/lib/__tests__/jmap-cross-account-move.test.ts
@@ -3,7 +3,7 @@ import { JMAPClient } from '../jmap/client';
 
 type Call = [string, Record<string, unknown>, string];
 
-function clientWithCopyResult(copyResult: Record<string, unknown>) {
+function clientWithCopyResult(copyResult: Record<string, unknown>, keywords: Record<string, boolean> = { $seen: true }) {
   const client = new JMAPClient('https://jmap.example.com', 'user@example.com', 'pass');
   Object.assign(client, { accountId: 'me' });
   const calls: Call[] = [];
@@ -11,7 +11,7 @@ function clientWithCopyResult(copyResult: Record<string, unknown>) {
     .mockImplementation(async (reqCalls: Call[]) => {
       const [method, args, id] = reqCalls[0];
       calls.push(reqCalls[0]);
-      if (method === 'Email/get') return { methodResponses: [['Email/get', { list: [{ id: 'm1', keywords: { $seen: true } }] }, id]] };
+      if (method === 'Email/get') return { methodResponses: [['Email/get', { list: [{ id: 'm1', keywords }] }, id]] };
       if (method === 'Email/copy') return { methodResponses: [['Email/copy', copyResult, id]] };
       if (method === 'Email/set') return { methodResponses: [['Email/set', { destroyed: args.destroy }, id]] };
       throw new Error(`unexpected ${method}`);
@@ -34,4 +34,17 @@ describe('copyEmailAcrossAccounts', () => {
     await expect(client.copyEmailAcrossAccounts('m1', 'me', 'grp', 'inbox')).rejects.toThrow('overQuota');
     expect(calls.some(([m]) => m === 'Email/set')).toBe(false);
   });
+
+  // Stalwart 1.0 builds the copy's keywords, and from them its hasAttachment,
+  // only from what the create sends.
+  it('carries the attachment keyword over to the copy on Stalwart 1.0', async () => {
+    const { client, calls } = clientWithCopyResult(
+      { created: { c: { id: 'new1' } } },
+      { $seen: true, '$label:work': true, $hasattachment: true },
+    );
+    await client.copyEmailAcrossAccounts('m1', 'me', 'grp', 'inbox');
+
+    const create = (calls.find(([m]) => m === 'Email/copy')![1].create as Record<string, Record<string, unknown>>).c;
+    expect(create.keywords).toEqual({ $seen: true, '$label:work': true, $hasattachment: true });
+  });
 });
diff --git a/lib/__tests__/jmap-keyword-account.test.ts b/lib/__tests__/jmap-keyword-account.test.ts
index cdf4c6956..4bd4504cc 100644
--- a/lib/__tests__/jmap-keyword-account.test.ts
+++ b/lib/__tests__/jmap-keyword-account.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { JMAPClient } from '../jmap/client';
+import { jmapErrorProperties, jmapErrorType } from '../jmap/jmap-error';
 
 // Regression coverage for the shared-account keyword write path (#281): keyword
 // mutations (tags, $answered/$forwarded) on a unified-inbox message must target
@@ -82,6 +83,15 @@ describe('Email/set clears keywords with null, not false', () => {
   beforeEach(() => vi.restoreAllMocks());
   afterEach(() => vi.restoreAllMocks());
 
+  it('updateEmailKeywords writes a full map of true values only', async () => {
+    // Local copies carry `$seen: false` after marking unread; RFC 8621 allows
+    // only true in a keyword map.
+    const client = createClient();
+    const { captured } = mockEmailSet();
+    await client.updateEmailKeywords('email-x', { $seen: false, $flagged: false, '$label:work': true, $pinned: true });
+    expect(patchFor(captured[0], 'email-x')).toEqual({ keywords: { '$label:work': true, $pinned: true } });
+  });
+
   it('markAsRead(false) removes $seen with null', async () => {
     const client = createClient();
     const { captured } = mockEmailSet();
@@ -169,3 +179,100 @@ describe('spam / not-spam patch keywords alongside the move (#850)', () => {
     expect(patch['keywords/$seen']).toBeUndefined();
   });
 });
+
+// Stalwart refuses a keyword per message in Email/set `notUpdated`, not as a
+// request error, so a writer that only awaited the request reported success
+// and the UI showed a tag that was gone after a reload. 1.0 refuses keywords
+// over 128 bytes and more than 100 keywords on one message; 0.16 accepts both
+// (it cuts long keywords to 128 characters).
+describe('keyword writes report what the server refused', () => {
+  beforeEach(() => vi.restoreAllMocks());
+  afterEach(() => vi.restoreAllMocks());
+
+  const TOO_LONG = {
+    type: 'invalidProperties',
+    description: 'Keywords cannot be longer than 128 bytes.',
+    properties: ['keywords'],
+  };
+  const TOO_MANY = {
+    type: 'tooManyKeywords',
+    description: 'An email cannot have more than 100 keywords.',
+    properties: ['keywords'],
+  };
+
+  /** Answers every Email/set with `result` for the ids it names. */
+  function mockEmailSetResult(result: (ids: string[]) => Record<string, unknown>) {
+    const captured: JMAPMethodCall[] = [];
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      captured.push(...body.methodCalls);
+      const methodResponses = body.methodCalls.map((call) => {
+        const ids = Object.keys((call[1].update ?? {}) as Record<string, unknown>);
+        return ['Email/set', { accountId: call[1].accountId, newState: 's2', ...result(ids) }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+    return captured;
+  }
+
+  const refuseAll = (error: Record<string, unknown>) => (ids: string[]) => ({
+    notUpdated: Object.fromEntries(ids.map((id) => [id, error])),
+  });
+  const acceptAll = (ids: string[]) => ({ updated: Object.fromEntries(ids.map((id) => [id, null])) });
+
+  const writers: Array<[string, (client: JMAPClient) => Promise<void>]> = [
+    ['updateEmailKeywords', (c) => c.updateEmailKeywords('m1', { [`$label:${'x'.repeat(130)}`]: true })],
+    ['setKeyword', (c) => c.setKeyword('m1', `$label:${'x'.repeat(130)}`)],
+    ['removeKeyword', (c) => c.removeKeyword('m1', '$label:work')],
+    ['batchUpdateKeywords', (c) => c.batchUpdateKeywords(['m1'], { [`keywords/$label:${'x'.repeat(130)}`]: true })],
+    ['toggleStar', (c) => c.toggleStar('m1', true)],
+    ['markAsRead', (c) => c.markAsRead('m1', true)],
+    ['batchMarkAsRead', (c) => c.batchMarkAsRead(['m1'], true)],
+  ];
+
+  it.each(writers)('%s rejects with the SetError type on Stalwart 1.0', async (_name, write) => {
+    mockEmailSetResult(refuseAll(TOO_LONG));
+    const error = await write(createClient()).then(() => null, (e: unknown) => e);
+    expect(error).toBeInstanceOf(Error);
+    expect((error as Error).message).toContain('Keywords cannot be longer than 128 bytes.');
+    expect(jmapErrorType(error)).toBe('invalidProperties');
+    expect(jmapErrorProperties(error)).toEqual(['keywords']);
+  });
+
+  it.each(writers)('%s resolves when Stalwart 0.16 accepts the write', async (_name, write) => {
+    mockEmailSetResult(acceptAll);
+    await expect(write(createClient())).resolves.toBeUndefined();
+  });
+
+  it('reports too many keywords on one message (Stalwart 1.0)', async () => {
+    mockEmailSetResult(refuseAll(TOO_MANY));
+    const error = await createClient().setKeyword('m1', '$label:one-more').then(() => null, (e: unknown) => e);
+    expect(jmapErrorType(error)).toBe('tooManyKeywords');
+    expect((error as Error).message).toContain('An email cannot have more than 100 keywords.');
+  });
+
+  it('reports a method-level refusal with its type', async () => {
+    vi.spyOn(globalThis, 'fetch').mockResolvedValue(new Response(JSON.stringify({
+      methodResponses: [['error', { type: 'accountReadOnly' }, '0']],
+    }), { status: 200 }));
+    const error = await createClient().toggleStar('m1', false).then(() => null, (e: unknown) => e);
+    expect(jmapErrorType(error)).toBe('accountReadOnly');
+  });
+
+  it('writes every batch before reporting a refused message', async () => {
+    const client = createClient();
+    Object.assign(client, { capabilities: { 'urn:ietf:params:jmap:core': { maxObjectsInSet: 1 } } });
+    const captured = mockEmailSetResult((ids) => (ids[0] === 'm1' ? refuseAll(TOO_LONG)(ids) : acceptAll(ids)));
+
+    await expect(client.batchUpdateKeywords(['m1', 'm2', 'm3'], { 'keywords/$label:work': true }))
+      .rejects.toThrow('Keywords cannot be longer than 128 bytes.');
+    expect(captured.map((call) => Object.keys(call[1].update as Record<string, unknown>))).toEqual([['m1'], ['m2'], ['m3']]);
+
+    captured.length = 0;
+    await expect(client.batchMarkAsRead(['m1', 'm2'], true)).rejects.toThrow(/mark as read/);
+    expect(captured).toHaveLength(2);
+  });
+});
diff --git a/lib/__tests__/jmap-keyword-discovery.test.ts b/lib/__tests__/jmap-keyword-discovery.test.ts
index 08b27e84c..29596d140 100644
--- a/lib/__tests__/jmap-keyword-discovery.test.ts
+++ b/lib/__tests__/jmap-keyword-discovery.test.ts
@@ -273,4 +273,17 @@ describe('JMAPClient.discoverKeywords', () => {
     ]);
     expect(result).toMatchObject({ scanned: 1, total: 1, complete: true });
   });
+
+  it('leaves the attachment keywords Stalwart 1.0 sets on every message out of the fallback labels', async () => {
+    const client = await connectedClient();
+    recordRequests(() => page(['m1', 'm2'], [
+      { '$label:work': true, $hasattachment: true },
+      { $seen: true, $hasnoattachment: true },
+    ], 2));
+
+    const result = await client.getKeywords();
+
+    expect(result.keywords).toEqual({ '$label:work': 1, $seen: 1 });
+    expect(result.labels.map((label) => label.id)).toEqual(['$label:work', '$seen']);
+  });
 });
diff --git a/lib/__tests__/keyword-nesting.test.ts b/lib/__tests__/keyword-nesting.test.ts
index 888c45ad5..c5e8f667e 100644
--- a/lib/__tests__/keyword-nesting.test.ts
+++ b/lib/__tests__/keyword-nesting.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect } from "vitest";
 import {
   MAX_KEYWORD_ID_LENGTH,
+  MAX_KEYWORD_LENGTH,
   buildKeywordTree,
   composeKeywordId,
   countKeywordNodes,
@@ -96,9 +97,17 @@ describe("hasChildKeywords", () => {
 });
 
 describe("MAX_KEYWORD_ID_LENGTH", () => {
-  it("leaves room for the `$label:` prefix within the 255-character keyword limit", () => {
-    expect(MAX_KEYWORD_ID_LENGTH).toBe(248);
-    expect("$label:".length + MAX_KEYWORD_ID_LENGTH).toBe(255);
+  // Stalwart 1.0 refuses a keyword over 128 bytes; 0.16 cuts it to 128
+  // characters, which detaches it from its tag.
+  it("leaves room for the `$label:` prefix within Stalwart's 128-byte keyword limit", () => {
+    expect(MAX_KEYWORD_LENGTH).toBe(128);
+    expect(MAX_KEYWORD_ID_LENGTH).toBe(121);
+    expect("$label:".length + MAX_KEYWORD_ID_LENGTH).toBe(128);
+  });
+
+  it("counts bytes, which a composed id always has as many of as characters", () => {
+    const id = composeKeywordId("work/clients", "Ärger & Überstunden");
+    expect(new TextEncoder().encode(`$label:${id}`).length).toBe(`$label:${id}`.length);
   });
 });
 
diff --git a/lib/__tests__/message-list-tabs-store.test.ts b/lib/__tests__/message-list-tabs-store.test.ts
index e61367dcf..5203bc828 100644
--- a/lib/__tests__/message-list-tabs-store.test.ts
+++ b/lib/__tests__/message-list-tabs-store.test.ts
@@ -1,12 +1,17 @@
 // Contract tests for the plugin-facing message-list category tabs
 // (api.tabs.set → stores/message-list-tabs-store.ts).
 
-import { describe, it, expect, beforeEach } from 'vitest';
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import {
   useMessageListTabsStore,
   validateTabsConfig,
   resolveTabFilter,
 } from '@/stores/message-list-tabs-store';
+import { useEmailStore } from '@/stores/email-store';
+import { messageListTabHooks } from '@/lib/plugin-hooks';
+import { jmapError } from '@/lib/jmap/jmap-error';
+import type { IJMAPClient } from '@/lib/jmap/client-interface';
+import type { Email } from '@/lib/jmap/types';
 import type { MessageListTab } from '@/lib/plugin-types';
 
 const GMAIL_STYLE = {
@@ -59,6 +64,17 @@ describe('validateTabsConfig', () => {
     })).toThrow(/reserved/);
   });
 
+  // Stalwart 1.0 sets one of these on every message and derives hasAttachment
+  // from it; a tab move would add or strip it.
+  it.each(['$hasattachment', '$hasnoattachment', '$HasAttachment'])(
+    'rejects the server-set attachment keyword %s (Stalwart 1.0)',
+    (keyword) => {
+      expect(() => validateTabsConfig({
+        tabs: [{ id: 'a', label: 'A', keyword }, { id: 'b', label: 'B' }],
+      })).toThrow(/reserved keyword "\$has(no)?attachment"/);
+    },
+  );
+
   it('rejects duplicate ids, multiple default tabs, and single-tab configs', () => {
     expect(() => validateTabsConfig({
       tabs: [{ id: 'a', label: 'A' }, { id: 'a', label: 'B', keyword: '$x' }],
@@ -166,3 +182,59 @@ describe('store registration', () => {
     expect(useMessageListTabsStore.getState().getCategoryFilter('inbox')).toEqual({ hasKeyword: '$category-promotions' });
   });
 });
+
+describe('categorizeEmails', () => {
+  const email = (id: string, accountId?: string) => ({
+    id, threadId: `t-${id}`, keywords: { $seen: true }, mailboxIds: { inbox: true },
+    from: [{ email: `${id}@example.com` }], accountId,
+  }) as unknown as Email;
+
+  beforeEach(() => {
+    useEmailStore.setState({ emails: [email('m1'), email('m2', 'shared')], mailboxes: [], selectedMailbox: '' });
+    useMessageListTabsStore.getState().registerTabs('category-tabs', GMAIL_STYLE);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    useEmailStore.setState({ emails: [] });
+  });
+
+  it('patches the category keywords and moves the messages locally', async () => {
+    const batchUpdateKeywords = vi.fn().mockResolvedValue(undefined);
+    const emitted = vi.spyOn(messageListTabHooks.onEmailCategorize, 'emit');
+
+    await expect(useMessageListTabsStore.getState()
+      .categorizeEmails({ batchUpdateKeywords } as unknown as IJMAPClient, ['m1'], 'promotions')).resolves.toBe(true);
+
+    expect(batchUpdateKeywords).toHaveBeenCalledWith(
+      ['m1'],
+      { 'keywords/$category-updates': null, 'keywords/$category-promotions': true },
+      undefined,
+    );
+    expect(useEmailStore.getState().emails[0].keywords).toEqual({ $seen: true, '$category-promotions': true });
+    expect(emitted).toHaveBeenCalledTimes(1);
+  });
+
+  it('rejects when Stalwart 1.0 refuses the keyword, leaving the refused messages as they were', async () => {
+    // What batchUpdateKeywords throws for a `notUpdated` entry on 1.0, here
+    // for a message that already has 100 keywords.
+    const refusal = jmapError('Failed to update keywords: An email cannot have more than 100 keywords.', {
+      type: 'tooManyKeywords', description: 'An email cannot have more than 100 keywords.', properties: ['keywords'],
+    });
+    const batchUpdateKeywords = vi.fn(async (_ids: string[], _patch: unknown, accountId?: string) => {
+      if (accountId === 'shared') throw refusal;
+    });
+    const emitted = vi.spyOn(messageListTabHooks.onEmailCategorize, 'emit');
+
+    await expect(useMessageListTabsStore.getState()
+      .categorizeEmails({ batchUpdateKeywords } as unknown as IJMAPClient, ['m1', 'm2'], 'promotions')).rejects.toBe(refusal);
+
+    // Both accounts were written; only the one the server accepted moved.
+    expect(batchUpdateKeywords).toHaveBeenCalledTimes(2);
+    const [m1, m2] = useEmailStore.getState().emails;
+    expect(m1.keywords).toEqual({ $seen: true, '$category-promotions': true });
+    expect(m2.keywords).toEqual({ $seen: true });
+    // No per-sender override is recorded for a move that did not happen.
+    expect(emitted).not.toHaveBeenCalled();
+  });
+});
diff --git a/lib/__tests__/plugin-api-gates.test.ts b/lib/__tests__/plugin-api-gates.test.ts
index b267c495f..074048138 100644
--- a/lib/__tests__/plugin-api-gates.test.ts
+++ b/lib/__tests__/plugin-api-gates.test.ts
@@ -304,4 +304,30 @@ describe('native keyword extension API', () => {
       expect(error).toMatch(/No active session/);
     }
   });
+
+  // Stalwart 1.0 derives hasAttachment from these, so a plugin adding or
+  // removing one would misreport the message.
+  it('keeps plugins off the attachment keywords Stalwart 1.0 sets itself', async () => {
+    const setKeyword = vi.fn();
+    const removeKeyword = vi.fn();
+    const updateEmailKeywords = vi.fn();
+    useAuthStore.setState({ client: { setKeyword, removeKeyword, updateEmailKeywords } as never });
+    const writer = plugin({ permissions: ['email:write'], grantedPermissions: ['email:write'] });
+
+    for (const method of ['jmap.setKeyword', 'jmap.removeKeyword', 'email.setKeyword', 'email.removeKeyword']) {
+      for (const keyword of ['$hasattachment', '$hasnoattachment', '$HasAttachment']) {
+        expect(await gateError(writer, method, ['email-1', keyword])).toMatch(/is set by the server/);
+      }
+    }
+    expect(await gateError(writer, 'jmap.setKeywords', ['email-1', { '$seen': true, '$hasattachment': true }]))
+      .toMatch(/"\$hasattachment" is set by the server/);
+
+    expect(setKeyword).not.toHaveBeenCalled();
+    expect(removeKeyword).not.toHaveBeenCalled();
+    expect(updateEmailKeywords).not.toHaveBeenCalled();
+
+    // Other keywords still go through.
+    await dispatchApiCall(writer, 'jmap.setKeyword', ['email-1', '$label:work']);
+    expect(setKeyword).toHaveBeenCalledWith('email-1', '$label:work', undefined);
+  });
 });
diff --git a/lib/__tests__/server-keywords.test.ts b/lib/__tests__/server-keywords.test.ts
new file mode 100644
index 000000000..bf4f62846
--- /dev/null
+++ b/lib/__tests__/server-keywords.test.ts
@@ -0,0 +1,66 @@
+import { describe, it, expect } from 'vitest';
+import type { Email } from '@/lib/jmap/types';
+import { generateEmailSource } from '@/lib/email-source';
+import { emailToReadView } from '@/lib/plugin-projection';
+import { isServerSetKeyword } from '@/lib/jmap/server-keywords';
+
+// Stalwart 1.0 puts `$hasattachment` or `$hasnoattachment` on every message and
+// derives hasAttachment from it. Where keywords are listed they would only
+// repeat hasAttachment, so they are left out; 0.16 never sets them.
+
+function makeEmail(keywords: Record<string, boolean>, hasAttachment: boolean): Email {
+  return {
+    id: 'm1',
+    threadId: 't1',
+    mailboxIds: { inbox: true },
+    keywords,
+    size: 2048,
+    receivedAt: '2026-09-25T10:00:00Z',
+    from: [{ name: 'Ann', email: 'ann@example.org' }],
+    to: [{ name: '', email: 'bob@example.org' }],
+    subject: 'Report',
+    preview: 'See attached',
+    hasAttachment,
+    textBody: [{ partId: '1', blobId: 'b1', size: 12, type: 'text/plain' }],
+    htmlBody: [],
+    bodyValues: { '1': { value: 'See attached', isEncodingProblem: false, isTruncated: false } },
+  } as unknown as Email;
+}
+
+describe('isServerSetKeyword', () => {
+  it('matches the two attachment keywords in any case, and nothing else', () => {
+    expect(isServerSetKeyword('$hasattachment')).toBe(true);
+    expect(isServerSetKeyword('$hasnoattachment')).toBe(true);
+    expect(isServerSetKeyword('$HasAttachment')).toBe(true);
+    expect(isServerSetKeyword('$seen')).toBe(false);
+    expect(isServerSetKeyword('$label:hasattachment')).toBe(false);
+  });
+});
+
+describe('server-set keywords in listings', () => {
+  it('leaves them out of the view-source keyword line on Stalwart 1.0', () => {
+    const source = generateEmailSource(makeEmail({ $seen: true, '$label:work': true, $hasattachment: true }, true));
+    expect(source).toContain('Has Attachment: Yes\n');
+    expect(source).toContain('Keywords: $seen, $label:work\n');
+    expect(source).not.toContain('$hasattachment');
+  });
+
+  it('drops the keyword line when the attachment keyword is the only one on Stalwart 1.0', () => {
+    const source = generateEmailSource(makeEmail({ $hasnoattachment: true }, false));
+    expect(source).toContain('Has Attachment: No\n');
+    expect(source).not.toContain('Keywords:');
+  });
+
+  it('keeps the plugin projection keywords to what the user set, with hasAttachment intact', () => {
+    const view = emailToReadView(makeEmail({ $seen: true, $hasattachment: true, $flagged: false }, true));
+    expect(view.keywords).toEqual(['$seen']);
+    expect(view.hasAttachment).toBe(true);
+    expect(view.source).not.toContain('$hasattachment');
+  });
+
+  it('lists keywords unchanged on Stalwart 0.16, which never sets them', () => {
+    const email = makeEmail({ $seen: true, '$label:work': true }, true);
+    expect(emailToReadView(email).keywords).toEqual(['$seen', '$label:work']);
+    expect(generateEmailSource(email)).toContain('Keywords: $seen, $label:work\n');
+  });
+});
diff --git a/lib/__tests__/thread-utils.test.ts b/lib/__tests__/thread-utils.test.ts
index 095142a7a..ea1acbd5a 100644
--- a/lib/__tests__/thread-utils.test.ts
+++ b/lib/__tests__/thread-utils.test.ts
@@ -10,6 +10,7 @@ import {
   getThreadTagIds,
   threadKeyFor,
   threadIdFromKey,
+  toggleTagKeywords,
 } from '../thread-utils';
 import type { Email, ThreadGroup } from '../jmap/types';
 
@@ -306,6 +307,41 @@ describe('getEmailTagIds', () => {
   });
 });
 
+// The result is written as the message's whole keyword map, where RFC 8621
+// allows only true values: a removed tag must be missing, not false.
+describe('toggleTagKeywords', () => {
+  it('adds a tag without disturbing the other keywords', () => {
+    expect(toggleTagKeywords({ $seen: true, '$label:red': true }, 'work'))
+      .toEqual({ $seen: true, '$label:red': true, '$label:work': true });
+  });
+
+  it('takes a set tag off by leaving its key out, under either prefix', () => {
+    const next = toggleTagKeywords({ $seen: true, '$label:red': true, '$color:red': true, '$label:work': true }, 'red');
+    expect(next).toEqual({ $seen: true, '$label:work': true });
+    expect(Object.values(next).every((value) => value === true)).toBe(true);
+  });
+
+  it('clears every tag, whatever the prefix, and keeps the rest', () => {
+    expect(toggleTagKeywords({ $flagged: true, '$label:red': true, '$color:blue': true }, null))
+      .toEqual({ $flagged: true });
+  });
+
+  it('drops keywords the local copy only marks false', () => {
+    // Marking a message unread or unstarred leaves these behind locally.
+    expect(toggleTagKeywords({ $seen: false, $flagged: false, '$label:red': false }, 'red'))
+      .toEqual({ '$label:red': true });
+  });
+
+  it('keeps the server-set attachment keyword Stalwart 1.0 reports', () => {
+    expect(toggleTagKeywords({ $hasattachment: true }, 'work')).toEqual({ $hasattachment: true, '$label:work': true });
+  });
+
+  it('starts from nothing when the message has no keywords', () => {
+    expect(toggleTagKeywords(undefined, 'work')).toEqual({ '$label:work': true });
+    expect(toggleTagKeywords(undefined, null)).toEqual({});
+  });
+});
+
 describe('getEmailTagId', () => {
   it('returns label from $label: keyword', () => {
     expect(getEmailTagId({ '$label:red': true, $seen: true })).toBe('red');
diff --git a/lib/email-source.ts b/lib/email-source.ts
index 7a80a2f12..efc9e4e85 100644
--- a/lib/email-source.ts
+++ b/lib/email-source.ts
@@ -4,6 +4,7 @@
 // exact same text the UI shows. Pure: depends only on the passed `email`.
 
 import type { Email } from '@/lib/jmap/types';
+import { isServerSetKeyword } from '@/lib/jmap/server-keywords';
 import { formatFileSize } from '@/lib/utils';
 
 export function generateEmailSource(email: Email): string {
@@ -66,8 +67,9 @@ export function generateEmailSource(email: Email): string {
   source += `Size: ${formatFileSize(email.size)}\n`;
   source += `Has Attachment: ${email.hasAttachment ? 'Yes' : 'No'}\n`;
   if (email.keywords) {
+    // Stalwart 1.0's attachment keywords only restate the line above.
     const keywords = Object.entries(email.keywords)
-      .filter(([_, v]) => v)
+      .filter(([k, v]) => v && !isServerSetKeyword(k))
       .map(([k]) => k)
       .join(', ');
     if (keywords) source += `Keywords: ${keywords}\n`;
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 870edbbe1..c877665f5 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -19,6 +19,7 @@ import { fileNodeDepth } from "./filenode-depth";
 import { contactFromWire, contactToWire } from "./contact-wire";
 import { SchedulingDeniedError } from "./scheduling-error";
 import { jmapError, jmapErrorType, jmapSetError, type JmapErrorInfo } from "./jmap-error";
+import { isServerSetKeyword } from "./server-keywords";
 
 export { SchedulingDeniedError };
 import { acceptedFileName, fileNameRulesFrom, type FileNameRules } from "@/lib/file-name-rules";
@@ -2346,7 +2347,12 @@ export class JMAPClient implements IJMAPClient {
 
     if (!supportsKeywordGet) {
       const scan = await this.discoverKeywords(options);
-      const labels: KeywordInfo[] = Object.entries(scan.keywords).map(([id, total]) => ({
+      // Stalwart 1.0 sets an attachment keyword on every message; it is not a
+      // label anyone applied.
+      const keywords = Object.fromEntries(
+        Object.entries(scan.keywords).filter(([id]) => !isServerSetKeyword(id)),
+      );
+      const labels: KeywordInfo[] = Object.entries(keywords).map(([id, total]) => ({
         id,
         name: id.startsWith('$label:') ? id.slice('$label:'.length) : id,
         color: null,
@@ -2355,7 +2361,7 @@ export class JMAPClient implements IJMAPClient {
         isProviderLabel: false,
         source: 'message',
       }));
-      return { ...scan, labels };
+      return { ...scan, keywords, labels };
     }
 
     if (options?.signal?.aborted) {
@@ -2571,10 +2577,16 @@ export class JMAPClient implements IJMAPClient {
     }
   }
 
+  // The keyword writers below check the Email/set outcome: the server refuses
+  // a keyword per message in `notUpdated`, not as a request error. Stalwart
+  // 1.0 refuses keywords over 128 bytes (invalidProperties) and more than 100
+  // keywords on one message (tooManyKeywords); a read-only shared folder
+  // refuses any. Unchecked, the UI showed the change until the next reload.
+
   async markAsRead(emailId: string, read: boolean = true, accountId?: string): Promise<void> {
     const targetAccountId = accountId || this.accountId;
 
-    await this.request([
+    const response = await this.request([
       ["Email/set", {
         accountId: targetAccountId,
         update: {
@@ -2584,21 +2596,28 @@ export class JMAPClient implements IJMAPClient {
         },
       }, "0"],
     ]);
+    this.assertEmailSetSucceeded(response, read ? "mark as read" : "mark as unread");
   }
 
   async batchMarkAsRead(emailIds: string[], read: boolean = true, accountId?: string): Promise<void> {
     if (emailIds.length === 0) return;
 
+    const responses: JMAPResponse[] = [];
     for (const batch of batched(emailIds, this.getMaxObjectsInSet())) {
       const updates = Object.fromEntries(batch.map(id => [id, { "keywords/$seen": read ? true : null }]));
-      await this.request([
+      responses.push(await this.request([
         ["Email/set", { accountId: accountId || this.accountId, update: updates }, "0"],
-      ]);
+      ]));
+    }
+    // Every batch is sent before a refusal is reported: one refused message
+    // must not leave the rest of the selection unchanged.
+    for (const response of responses) {
+      this.assertEmailSetSucceeded(response, read ? "mark as read" : "mark as unread");
     }
   }
 
   async toggleStar(emailId: string, starred: boolean, accountId?: string): Promise<void> {
-    await this.request([
+    const response = await this.request([
       ["Email/set", {
         accountId: accountId || this.accountId,
         update: {
@@ -2608,23 +2627,29 @@ export class JMAPClient implements IJMAPClient {
         },
       }, "0"],
     ]);
+    this.assertEmailSetSucceeded(response, starred ? "star the email" : "unstar the email");
   }
 
   async updateEmailKeywords(emailId: string, keywords: Record<string, boolean>, accountId?: string): Promise<void> {
-    await this.request([
+    // Every value of a full keyword map must be true (RFC 8621 section
+    // 4.1.1). Callers start from local copies, where marking a message unread
+    // or unstarred leaves `$seen: false` / `$flagged: false` behind.
+    const keywordSet = Object.fromEntries(Object.entries(keywords).filter(([, value]) => value === true));
+    const response = await this.request([
       ["Email/set", {
         accountId: accountId || this.accountId,
         update: {
           [emailId]: {
-            keywords,
+            keywords: keywordSet,
           },
         },
       }, "0"],
     ]);
+    this.assertEmailSetSucceeded(response, "update keywords");
   }
 
   async setKeyword(emailId: string, keyword: string, accountId?: string): Promise<void> {
-    await this.request([
+    const response = await this.request([
       ["Email/set", {
         accountId: accountId || this.accountId,
         update: {
@@ -2634,10 +2659,11 @@ export class JMAPClient implements IJMAPClient {
         },
       }, "0"],
     ]);
+    this.assertEmailSetSucceeded(response, `set ${keyword}`);
   }
 
   async removeKeyword(emailId: string, keyword: string, accountId?: string): Promise<void> {
-    await this.request([
+    const response = await this.request([
       ["Email/set", {
         accountId: accountId || this.accountId,
         update: {
@@ -2647,6 +2673,7 @@ export class JMAPClient implements IJMAPClient {
         },
       }, "0"],
     ]);
+    this.assertEmailSetSucceeded(response, `remove ${keyword}`);
   }
 
   /**
@@ -2656,11 +2683,15 @@ export class JMAPClient implements IJMAPClient {
    */
   async batchUpdateKeywords(emailIds: string[], patch: Record<string, boolean | null>, accountId?: string): Promise<void> {
     if (emailIds.length === 0 || Object.keys(patch).length === 0) return;
+    const responses: JMAPResponse[] = [];
     for (const batch of batched(emailIds, this.getMaxObjectsInSet())) {
       const update = Object.fromEntries(batch.map(id => [id, { ...patch }]));
-      await this.request([
+      responses.push(await this.request([
         ["Email/set", { accountId: accountId || this.accountId, update }, "0"],
-      ]);
+      ]));
+    }
+    for (const response of responses) {
+      this.assertEmailSetSucceeded(response, "update keywords");
     }
   }
 
@@ -2797,9 +2828,9 @@ export class JMAPClient implements IJMAPClient {
   private assertEmailSetSucceeded(response: JMAPResponse, action: string): void {
     const [name, result] = response.methodResponses?.[0] ?? [];
     if (name === "error") {
-      throw new Error(`Failed to ${action}: ${result?.description || result?.type || 'unknown error'}`);
+      throw jmapError(`Failed to ${action}: ${result?.description || result?.type || 'unknown error'}`, result);
     }
-    const failed: Record<string, { type?: string; description?: string }> = {
+    const failed: Record<string, JmapErrorInfo> = {
       ...(result?.notUpdated ?? {}),
       ...(result?.notDestroyed ?? {}),
     };
@@ -2807,10 +2838,11 @@ export class JMAPClient implements IJMAPClient {
     if (failedIds.length === 0) return;
     const first = failed[failedIds[0]];
     const reason = first?.description || first?.type || 'unknown error';
-    throw new Error(
+    throw jmapError(
       failedIds.length === 1
         ? `Failed to ${action}: ${reason}`
         : `Failed to ${action} ${failedIds.length} email(s); first error: ${reason}`,
+      first,
     );
   }
 
@@ -9251,7 +9283,9 @@ export class JMAPClient implements IJMAPClient {
     destMailboxId: string,
   ): Promise<string> {
     // Email/copy drops keywords unless the create sets them, so carry the
-    // source's over — otherwise the moved message shows up as unread.
+    // source's over — otherwise the moved message shows up as unread. All of
+    // them: Stalwart 1.0 also takes the copy's `$hasattachment` /
+    // `$hasnoattachment`, and with them its hasAttachment, only from here.
     const srcResp = await this.request([
       ["Email/get", { accountId: fromAccountId, ids: [emailId], properties: ["keywords"] }, "0"],
     ]);
diff --git a/lib/jmap/server-keywords.ts b/lib/jmap/server-keywords.ts
new file mode 100644
index 000000000..24b6e9267
--- /dev/null
+++ b/lib/jmap/server-keywords.ts
@@ -0,0 +1,18 @@
+/**
+ * Keywords the server sets on its own.
+ *
+ * Stalwart 1.0 puts `$hasattachment` or `$hasnoattachment` on every message it
+ * stores and derives Email `hasAttachment` (the property and the filter) from
+ * them alone. A patch can still add or remove them, which flips
+ * `hasAttachment`. So they are not tags: they are hidden wherever keywords are
+ * listed, and no plugin or category tab may add or remove them. A copy must
+ * carry them over, though: Email/copy takes a message's keywords only from the
+ * request, and a copy sent without them loses its attachment flag. Stalwart
+ * 0.16 never sets them.
+ */
+export const SERVER_SET_KEYWORDS: ReadonlySet<string> = new Set(['$hasattachment', '$hasnoattachment']);
+
+/** True for a keyword the server sets itself (keywords are case-insensitive). */
+export function isServerSetKeyword(keyword: string): boolean {
+  return SERVER_SET_KEYWORDS.has(keyword.toLowerCase());
+}
diff --git a/lib/keyword-nesting.ts b/lib/keyword-nesting.ts
index 57a4fc9b0..e8e2eef8c 100644
--- a/lib/keyword-nesting.ts
+++ b/lib/keyword-nesting.ts
@@ -11,7 +11,8 @@
  * RFC 8621 section 4.1.1 allows a keyword of 1-255 characters from the ASCII
  * range %x21-%x7e minus `( ) { ] % * " \`, so the separator is legal but the
  * length of a deep id is not free - `MAX_KEYWORD_ID_LENGTH` is the budget a
- * composed id has to stay within.
+ * composed id has to stay within. Stalwart caps keywords well below the RFC
+ * (see `MAX_KEYWORD_LENGTH`).
  *
  * Turning any of this into text for the screen lives in `keyword-format`.
  */
@@ -21,8 +22,13 @@ import { KEYWORD_PREFIX } from "./thread-utils";
 /** Separates parent from child inside a tag id. */
 export const KEYWORD_SEPARATOR = "/";
 
-/** Longest keyword a JMAP server has to accept (RFC 8621, section 4.1.1). */
-export const MAX_KEYWORD_LENGTH = 255;
+/**
+ * Longest keyword Stalwart keeps, in bytes (the RFC allows 255). 1.0 refuses a
+ * longer new keyword ("Keywords cannot be longer than 128 bytes.") and 0.16
+ * cuts it to 128 characters, which silently detaches it from its tag. Tag
+ * keywords are ASCII, so bytes and characters are the same count.
+ */
+export const MAX_KEYWORD_LENGTH = 128;
 
 /** What is left for the id once the `$label:` prefix is spent. */
 export const MAX_KEYWORD_ID_LENGTH = MAX_KEYWORD_LENGTH - KEYWORD_PREFIX.length;
diff --git a/lib/plugin-projection.ts b/lib/plugin-projection.ts
index 865df43e3..40dbf9590 100644
--- a/lib/plugin-projection.ts
+++ b/lib/plugin-projection.ts
@@ -5,6 +5,7 @@
 import type { Email } from '@/lib/jmap/types';
 import type { EmailReadView } from '@/lib/plugin-types';
 import { generateEmailSource } from '@/lib/email-source';
+import { isServerSetKeyword } from '@/lib/jmap/server-keywords';
 
 // Resolve a message's plain-text body from its JMAP body parts. Plugins that
 // translate or scan content need the real body, not just the short `preview`
@@ -57,7 +58,8 @@ export function emailToReadView(email: Email): EmailReadView {
     text: plainTextFromEmail(email),
     headers: email.headers,
     source: generateEmailSource(email),
-    keywords: Object.keys(email.keywords || {}).filter(k => email.keywords[k]),
+    // Stalwart 1.0's attachment keywords are what `hasAttachment` says.
+    keywords: Object.keys(email.keywords || {}).filter(k => email.keywords[k] && !isServerSetKeyword(k)),
     auth: email.authenticationResults,
   };
 }
diff --git a/lib/plugin-sandbox/host-api.ts b/lib/plugin-sandbox/host-api.ts
index 6393177e4..f423bfe0d 100644
--- a/lib/plugin-sandbox/host-api.ts
+++ b/lib/plugin-sandbox/host-api.ts
@@ -21,6 +21,7 @@ import type { MessageListTabsConfig } from '../plugin-types';
 import { apiFetch, getPathPrefix } from '../browser-navigation';
 import { reportUploadProgress } from '../upload-progress';
 import { DEFAULT_KEYWORD_SCAN_LIMIT } from '../jmap/client';
+import { isServerSetKeyword } from '../jmap/server-keywords';
 import { suggestKeywordColor } from '../keyword-discovery';
 import { MAX_KEYWORD_LENGTH } from '../keyword-nesting';
 import { KEYWORD_PREFIX } from '../thread-utils';
@@ -994,6 +995,11 @@ function assertPluginKeyword(keyword: unknown): string {
   if (typeof keyword !== 'string' || !PLUGIN_KEYWORD_RE.test(keyword)) {
     throw new Error(`Invalid JMAP keyword "${String(keyword)}"`);
   }
+  // Stalwart 1.0 derives hasAttachment from these; adding or removing one
+  // would misreport the message.
+  if (isServerSetKeyword(keyword)) {
+    throw new Error(`JMAP keyword "${keyword}" is set by the server`);
+  }
   return keyword;
 }
 
diff --git a/lib/thread-utils.ts b/lib/thread-utils.ts
index a3476c6ec..bb78a4eaf 100644
--- a/lib/thread-utils.ts
+++ b/lib/thread-utils.ts
@@ -231,6 +231,37 @@ export function getEmailTagIds(keywords: Record<string, boolean> | undefined): s
   return [...tags];
 }
 
+/**
+ * The complete keyword map after toggling one tag, or after clearing every tag
+ * (`tagId` null), for writing as a whole. Every value in such a map must be
+ * true (RFC 8621 section 4.1.1), so a tag comes off by leaving its key out,
+ * and so do keywords the local copy only marks false (`$seen: false` after
+ * marking a message unread). Both prefixes name the same tag, so taking a tag
+ * off clears whichever spellings are set.
+ */
+export function toggleTagKeywords(
+  keywords: Record<string, boolean> | undefined,
+  tagId: string | null,
+): Record<string, true> {
+  const next: Record<string, true> = {};
+  for (const [key, value] of Object.entries(keywords ?? {})) {
+    if (value === true) next[key] = true;
+  }
+  if (tagId === null) {
+    for (const key of Object.keys(next)) {
+      if (key.startsWith(KEYWORD_PREFIX) || key.startsWith(KEYWORD_PREFIX_LEGACY)) delete next[key];
+    }
+    return next;
+  }
+  const activeKeys = [KEYWORD_PREFIX + tagId, KEYWORD_PREFIX_LEGACY + tagId].filter((key) => next[key]);
+  if (activeKeys.length > 0) {
+    for (const key of activeKeys) delete next[key];
+  } else {
+    next[KEYWORD_PREFIX + tagId] = true;
+  }
+  return next;
+}
+
 /**
  * Gets the first tag id set on a message, if any.
  * Reads both the current $label: prefix and the legacy $color: prefix.
diff --git a/stores/__tests__/email-store-cross-account-move.test.ts b/stores/__tests__/email-store-cross-account-move.test.ts
index fcfdb9c26..5390ca62d 100644
--- a/stores/__tests__/email-store-cross-account-move.test.ts
+++ b/stores/__tests__/email-store-cross-account-move.test.ts
@@ -1,10 +1,14 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { useEmailStore } from '../email-store';
+import { useAuthStore } from '../auth-store';
 import type { Email, Mailbox } from '@/lib/jmap/types';
 import type { IJMAPClient } from '@/lib/jmap/client-interface';
 
 type Store = ReturnType<typeof useEmailStore.getState>;
 
+// The routing tests replace it with a spy; the carry-over test needs the real one.
+const realCrossAccountMoveEmails = useEmailStore.getState().crossAccountMoveEmails;
+
 function makeMailbox(overrides: Partial<Mailbox>): Mailbox {
   return {
     id: 'inbox',
@@ -100,3 +104,32 @@ describe('email-store moveToMailboxCrossAware', () => {
     );
   });
 });
+
+describe('email-store crossAccountMoveEmails between two servers', () => {
+  it('imports the message with every keyword, the Stalwart 1.0 attachment keyword included', async () => {
+    const blob = new Blob(['From: x@example.com\r\n\r\nhi']);
+    const keywords = { $seen: true, '$label:work': true, $hasattachment: true };
+    const source = {
+      getEmail: vi.fn().mockResolvedValue({ id: 'e1', blobId: 'blob-1', keywords }),
+      fetchBlob: vi.fn().mockResolvedValue(blob),
+      deleteEmail: vi.fn().mockResolvedValue(undefined),
+    };
+    const dest = { importRawEmail: vi.fn().mockResolvedValue('imported-1') };
+    useAuthStore.setState({
+      activeAccountId: 'login-A',
+      getClientForAccount: ((id: string) => (id === 'login-A' ? source : id === 'login-B' ? dest : undefined)) as never,
+    } as never);
+    useEmailStore.setState({
+      emails: [makeEmail('e1', 'srv-inbox-A')],
+      crossAccountMoveEmails: realCrossAccountMoveEmails,
+      // The background folder refresh after the move is not under test.
+      fetchMailboxes: vi.fn().mockResolvedValue(undefined) as unknown as Store['fetchMailboxes'],
+      fetchAccountMailboxes: vi.fn().mockResolvedValue(undefined) as unknown as Store['fetchAccountMailboxes'],
+    });
+
+    await useEmailStore.getState().crossAccountMoveEmails(new Map([['login-A', ['e1']]]), 'login-B', 'inbox-B');
+
+    expect(dest.importRawEmail).toHaveBeenCalledWith(blob, { 'inbox-B': true }, keywords, undefined);
+    expect(source.deleteEmail).toHaveBeenCalledWith('e1', undefined);
+  });
+});
diff --git a/stores/__tests__/email-store-shared-folder-keywords.test.ts b/stores/__tests__/email-store-shared-folder-keywords.test.ts
index 97659d646..5a494cb47 100644
--- a/stores/__tests__/email-store-shared-folder-keywords.test.ts
+++ b/stores/__tests__/email-store-shared-folder-keywords.test.ts
@@ -3,6 +3,7 @@ import { useEmailStore } from '../email-store';
 import { useAuthStore } from '../auth-store';
 import type { Email, Mailbox } from '@/lib/jmap/types';
 import type { IJMAPClient } from '@/lib/jmap/client-interface';
+import { jmapError } from '@/lib/jmap/jmap-error';
 
 // Regression coverage for keyword writes (tags, $pinned, $answered/$forwarded,
 // $mdnsent) performed while viewing a shared/group mailbox DIRECTLY from the
@@ -115,6 +116,19 @@ describe('non-unified shared-folder keyword routing', () => {
     );
   });
 
+  it('setEmailKeywords keeps the old tags when Stalwart 1.0 refuses the keyword', async () => {
+    // What updateEmailKeywords throws for a `notUpdated` entry on 1.0 (0.16
+    // accepts the write and cuts the keyword to 128 characters instead).
+    const refusal = jmapError('Failed to update keywords: Keywords cannot be longer than 128 bytes.', {
+      type: 'invalidProperties', properties: ['keywords'],
+    });
+    vi.mocked(activeClient.updateEmailKeywords).mockRejectedValueOnce(refusal);
+
+    await expect(useEmailStore.getState().setEmailKeywords(activeClient, 'e1', { [`$label:${'x'.repeat(130)}`]: true }))
+      .rejects.toBe(refusal);
+    expect(useEmailStore.getState().emails.find(e => e.id === 'e1')?.keywords).toEqual({});
+  });
+
   it('setEmailKeywords patches local state so the tag shows immediately', async () => {
     await useEmailStore.getState().setEmailKeywords(activeClient, 'e1', { '$label:work': true });
 
diff --git a/stores/email-store.ts b/stores/email-store.ts
index b333c06ec..aa9a9ce00 100644
--- a/stores/email-store.ts
+++ b/stores/email-store.ts
@@ -2671,6 +2671,8 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
               throw new Error('Source email has no raw blob to copy');
             }
             const blob = await sourceClient.fetchBlob(full.blobId, undefined, undefined, sourceJmapAccountId);
+            // Every keyword, the server's attachment ones included (see
+            // lib/jmap/server-keywords.ts).
             const keywords: Record<string, boolean> = { ...(full.keywords ?? {}) };
             await destClient.importRawEmail(blob, { [destMailboxId]: true }, keywords, destJmapAccountId);
             await sourceClient.deleteEmail(emailId, sourceJmapAccountId);
diff --git a/stores/message-list-tabs-store.ts b/stores/message-list-tabs-store.ts
index 5a95ba890..fc4575cfa 100644
--- a/stores/message-list-tabs-store.ts
+++ b/stores/message-list-tabs-store.ts
@@ -16,15 +16,19 @@ import type { MessageListTab, MessageListTabsConfig } from '@/lib/plugin-types';
 import { MAX_MESSAGE_LIST_TABS } from '@/lib/plugin-types';
 import { messageListTabHooks } from '@/lib/plugin-hooks';
 import { keywordPointer } from '@/lib/jmap/patch-pointer';
+import { SERVER_SET_KEYWORDS } from '@/lib/jmap/server-keywords';
 import { useEmailStore } from './email-store';
 
 // ─── Validation ──────────────────────────────────────────────
 
 // System keywords a tab may never claim: filtering the inbox by these would
-// hijack read/star/pin semantics, and `$label:` keywords belong to user tags.
+// hijack read/star/pin semantics, moving mail between tabs would add and
+// remove the server's own attachment keywords (flipping `hasAttachment`), and
+// `$label:` keywords belong to user tags.
 const RESERVED_KEYWORDS = new Set([
   '$seen', '$flagged', '$draft', '$answered', '$forwarded',
   '$recent', '$junk', '$notjunk', '$phishing', '$pinned',
+  ...SERVER_SET_KEYWORDS,
 ]);
 
 const TAB_ID_RE = /^[a-z0-9][a-z0-9_-]{0,31}$/i;
@@ -336,31 +340,45 @@ export const useMessageListTabsStore = create<MessageListTabsStore>()((set, get)
       list.push(id);
       byAccount.set(acct, list);
     }
+    // A refused write (a read-only shared folder; Stalwart 1.0 also refuses a
+    // 101st keyword on a message) fails the move: the plugin's promise rejects
+    // with the server's reason and the categorize hooks stay quiet. The other
+    // accounts are still written, and their messages still move locally.
+    const written: string[] = [];
+    let failure: unknown = null;
     for (const [acct, ids] of byAccount) {
-      await client.batchUpdateKeywords(ids, patch, acct);
+      try {
+        await client.batchUpdateKeywords(ids, patch, acct);
+        written.push(...ids);
+      } catch (err) {
+        failure ??= err;
+      }
     }
 
     // Optimistic local update: fix keywords in place, and drop moved messages
     // from a keyword-filtered view they no longer belong to. (Query-based
     // active tabs are left as-is — membership there is server-derived.)
-    const idSet = new Set(emailIds);
+    const idSet = new Set(written);
     const activeTab = tabs.find((t) => t.id === get().activeTabId);
     const shouldDrop = !!activeTab && !activeTab.query && activeTab.id !== toTabId
       && get().isEnabledForRole(
         emailState.mailboxes.find((mb) => mb.id === emailState.selectedMailbox)?.role ?? null,
       );
-    useEmailStore.setState((state) => ({
-      emails: state.emails
-        .map((e) => {
-          if (!idSet.has(e.id)) return e;
-          const keywords = { ...e.keywords };
-          for (const k of keywordsRemoved) delete keywords[k];
-          if (keywordAdded) keywords[keywordAdded] = true;
-          return { ...e, keywords };
-        })
-        .filter((e) => !shouldDrop || !idSet.has(e.id)),
-    }));
+    if (idSet.size > 0) {
+      useEmailStore.setState((state) => ({
+        emails: state.emails
+          .map((e) => {
+            if (!idSet.has(e.id)) return e;
+            const keywords = { ...e.keywords };
+            for (const k of keywordsRemoved) delete keywords[k];
+            if (keywordAdded) keywords[keywordAdded] = true;
+            return { ...e, keywords };
+          })
+          .filter((e) => !shouldDrop || !idSet.has(e.id)),
+      }));
+    }
 
+    if (failure) throw failure;
     void messageListTabHooks.onEmailCategorize.emit(ctx);
     return true;
   },