feat: share mail folders with other users via mail:share

components/layout/mailbox-context-menu.tsx

@@ -21,6 +21,7 @@ import {
   FolderX,
   RefreshCw,
   Upload,
+  Users,
 } from "lucide-react";
 
 interface Position {
@@ -85,6 +86,8 @@ interface MailboxContextMenuProps {
   onCreateFolder?: (accountId?: string) => void;
   onRenameFolder?: (mailboxId: string) => void;
   onDeleteFolder?: (mailboxId: string) => void;
+  /** Share the folder with other principals (mail:share); omitted when the server lacks it. */
+  onShareFolder?: (mailboxId: string) => void;
   onImportEmail?: (mailboxId: string) => void;
   onRefresh?: () => void;
 }
@@ -104,6 +107,7 @@ export function MailboxContextMenu({
   onCreateFolder,
   onRenameFolder,
   onDeleteFolder,
+  onShareFolder,
   onImportEmail,
   onRefresh,
 }: MailboxContextMenuProps) {
@@ -158,6 +162,9 @@ export function MailboxContextMenu({
   const canSetSeen = mailbox.myRights?.maySetSeen !== false;
   const canRemoveItems = mailbox.myRights?.mayRemoveItems !== false;
   const canAddItems = mailbox.myRights?.mayAddItems !== false;
+  // Sharing needs the mail:share extension (handler present) and, on a folder
+  // someone else shared with us, their permission to re-share it.
+  const canShare = !!onShareFolder && (!mailbox.isShared || mailbox.myRights?.mayShare === true);
 
   const fullPath = getMailboxPath(mailbox, mailboxes);
 
@@ -198,6 +205,15 @@ export function MailboxContextMenu({
         disabled={!onRenameFolder || !canRename}
         testId="mailbox-rename"
       />
+      {onShareFolder && (
+        <ContextMenuItem
+          icon={Users}
+          label={t("share")}
+          onClick={() => handleAction(() => onShareFolder(mailbox.id))}
+          disabled={!canShare}
+          testId="mailbox-share"
+        />
+      )}
 
       <ContextMenuSeparator />
 

components/layout/mailbox-share-dialog.tsx

@@ -0,0 +1,75 @@
+"use client";
+
+import { useCallback, useEffect, useState } from "react";
+import { Loader2 } from "lucide-react";
+import type { IJMAPClient } from "@/lib/jmap/client-interface";
+import type { Mailbox, MailboxRights } from "@/lib/jmap/types";
+import { ShareCollectionDialog } from "@/components/settings/share-collection-dialog";
+import { toast } from "@/stores/toast-store";
+import { useTranslations } from "next-intl";
+
+interface MailboxShareDialogProps {
+  client: IJMAPClient;
+  mailbox: Mailbox;
+  onClose: () => void;
+}
+
+/**
+ * Share a mail folder with other principals (urn:ietf:params:jmap:mail:share).
+ * The folder's current `shareWith` is loaded on open - it is not part of the
+ * regular folder list - and re-read after every change so the dialog always
+ * reflects what the server holds.
+ */
+export function MailboxShareDialog({ client, mailbox, onClose }: MailboxShareDialogProps) {
+  const t = useTranslations("sharing");
+  const [shareWith, setShareWith] = useState<Record<string, MailboxRights> | null | undefined>(undefined);
+
+  // A folder of a delegated account lives under its owner's accountId with
+  // the owner's (un-namespaced) mailbox id.
+  const accountId = mailbox.isShared ? mailbox.accountId : undefined;
+  const jmapMailboxId = mailbox.originalId || mailbox.id;
+
+  const load = useCallback(async () => {
+    if (!client.getMailboxShareWith) {
+      setShareWith(null);
+      return;
+    }
+    try {
+      setShareWith(await client.getMailboxShareWith(jmapMailboxId, accountId));
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : t("share_failed"));
+      onClose();
+    }
+  }, [client, jmapMailboxId, accountId, onClose, t]);
+
+  useEffect(() => {
+    void load();
+  }, [load]);
+
+  if (shareWith === undefined) {
+    return (
+      <div className="fixed inset-0 z-50 flex items-center justify-center">
+        <div className="absolute inset-0 bg-black/50 backdrop-blur-[1px]" onClick={onClose} aria-hidden="true" />
+        <div className="relative rounded-lg border border-border bg-background px-6 py-4 shadow-xl">
+          <Loader2 className="h-5 w-5 animate-spin text-muted-foreground" aria-label={t("loading_principals")} />
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <ShareCollectionDialog
+      client={client}
+      kind="mailbox"
+      collectionName={mailbox.name}
+      shareWith={shareWith}
+      ownAccountId={mailbox.accountId || client.getAccountId()}
+      onShare={async (principalId, rights) => {
+        if (!client.setMailboxShare) return;
+        await client.setMailboxShare(jmapMailboxId, principalId, rights as MailboxRights | null, accountId);
+        await load();
+      }}
+      onClose={onClose}
+    />
+  );
+}

components/layout/sidebar.tsx

@@ -88,6 +88,7 @@ interface SidebarProps {
   onCreateFolder?: (accountId?: string) => void;
   onRenameFolder?: (mailboxId: string) => void;
   onDeleteFolder?: (mailboxId: string) => void;
+  onShareFolder?: (mailboxId: string) => void;
   onImportEmail?: (mailboxId: string) => void;
   onRefreshMailboxes?: () => void;
   scheduledTotal?: number;
@@ -819,6 +820,7 @@ export function Sidebar({
   onCreateFolder,
   onRenameFolder,
   onDeleteFolder,
+  onShareFolder,
   onImportEmail,
   onRefreshMailboxes,
   scheduledTotal = 0,
@@ -1506,6 +1508,7 @@ export function Sidebar({
         onCreateFolder={onCreateFolder}
         onRenameFolder={onRenameFolder}
         onDeleteFolder={onDeleteFolder}
+        onShareFolder={onShareFolder}
         onImportEmail={onImportEmail}
         onRefresh={onRefreshMailboxes}
       />

components/mail/mail-app.tsx

@@ -32,6 +32,7 @@ import type { UnifiedAccountClient } from "@/lib/unified-mailbox";
 import { KeyboardShortcutsModal } from "@/components/keyboard-shortcuts-modal";
 import { useEmailStore, buildUnifiedAccountClients } from "@/stores/email-store";
 import { toast } from "@/stores/toast-store";
+import { MailboxShareDialog } from "@/components/layout/mailbox-share-dialog";
 import { useAuthStore, redirectToLogin, saveRedirectAfterLogin } from '@/stores/auth-store';
 import { useSettingsStore } from "@/stores/settings-store";
 import { useContactStore } from "@/stores/contact-store";
@@ -2719,6 +2720,16 @@ export function MailApp({ linkSegments }: MailAppProps = {}) {
     }
   };
 
+  // Folder sharing (mail:share): the dialog loads the folder's shareWith on
+  // open, so only the id needs to be remembered here.
+  const [sharingMailboxId, setSharingMailboxId] = useState<string | null>(null);
+  const sharingMailbox = sharingMailboxId ? mailboxes.find(mb => mb.id === sharingMailboxId) ?? null : null;
+  const supportsMailboxSharing = !!client?.supportsMailboxSharing?.();
+  const handleShareFolderFromContextMenu = (mailboxId: string) => {
+    if (!client) return;
+    setSharingMailboxId(mailboxId);
+  };
+
   const handleDeleteFolderFromContextMenu = async (mailboxId: string) => {
     if (!client) return;
     const mailbox = mailboxes.find(mb => mb.id === mailboxId);
@@ -3444,6 +3455,7 @@ export function MailApp({ linkSegments }: MailAppProps = {}) {
               onCreateFolder={handleCreateFolderFromContextMenu}
               onRenameFolder={handleRenameFolderFromContextMenu}
               onDeleteFolder={handleDeleteFolderFromContextMenu}
+              onShareFolder={supportsMailboxSharing ? handleShareFolderFromContextMenu : undefined}
               onImportEmail={handleImportEmailFromContextMenu}
               onRefreshMailboxes={handleRefreshMailboxes}
               onCompose={() => {
@@ -4182,6 +4194,13 @@ export function MailApp({ linkSegments }: MailAppProps = {}) {
         )}
         <ConfirmDialog {...confirmDialogProps} />
         <PromptDialog {...promptDialogProps} />
+        {client && sharingMailbox && (
+          <MailboxShareDialog
+            client={client}
+            mailbox={sharingMailbox}
+            onClose={() => setSharingMailboxId(null)}
+          />
+        )}
         <TotpReauthDialog />
       </div>
     </DragDropProvider>

components/settings/share-collection-dialog.tsx

@@ -6,11 +6,11 @@ import { Button } from "@/components/ui/button";
 import { Avatar } from "@/components/ui/avatar";
 import { X, Loader2, UserPlus, Trash2, Users, ChevronDown } from "lucide-react";
 import type { IJMAPClient } from "@/lib/jmap/client-interface";
-import type { Principal, CalendarRights, AddressBookRights, FileNodeRights } from "@/lib/jmap/types";
+import type { Principal, CalendarRights, AddressBookRights, FileNodeRights, MailboxRights } from "@/lib/jmap/types";
 import { toast } from "@/stores/toast-store";
 
-type ShareKind = "calendar" | "addressBook" | "file";
-type AnyRights = CalendarRights | AddressBookRights | FileNodeRights;
+type ShareKind = "calendar" | "addressBook" | "file" | "mailbox";
+type AnyRights = CalendarRights | AddressBookRights | FileNodeRights | MailboxRights;
 
 type RolePreset = "freeBusy" | "read" | "readWrite" | "manager" | "custom";
 
@@ -54,6 +54,37 @@ const FILE_PRESETS: Record<Exclude<RolePreset, "custom" | "freeBusy">, FileNodeR
   },
 };
 
+// Mail folder rights (RFC 8621 §2 + mail:share). "Read & write" lets the
+// grantee file, flag and remove mail; "Manager" additionally lets them
+// rename/delete the folder, create subfolders, send as it and re-share.
+const MAILBOX_PRESETS: Record<Exclude<RolePreset, "custom" | "freeBusy">, MailboxRights> = {
+  read: {
+    mayReadItems: true, mayAddItems: false, mayRemoveItems: false, maySetSeen: true,
+    maySetKeywords: false, mayCreateChild: false, mayRename: false, mayDelete: false,
+    maySubmit: false, mayShare: false,
+  },
+  readWrite: {
+    mayReadItems: true, mayAddItems: true, mayRemoveItems: true, maySetSeen: true,
+    maySetKeywords: true, mayCreateChild: false, mayRename: false, mayDelete: false,
+    maySubmit: false, mayShare: false,
+  },
+  manager: {
+    mayReadItems: true, mayAddItems: true, mayRemoveItems: true, maySetSeen: true,
+    maySetKeywords: true, mayCreateChild: true, mayRename: true, mayDelete: true,
+    maySubmit: true, mayShare: true,
+  },
+};
+
+function detectMailboxPreset(r: MailboxRights): RolePreset {
+  for (const [name, preset] of Object.entries(MAILBOX_PRESETS) as [Exclude<RolePreset, "custom" | "freeBusy">, MailboxRights][]) {
+    const keys = Object.keys(preset) as (keyof MailboxRights)[];
+    if (keys.every((k) => preset[k] === (r[k] ?? false))) {
+      return name;
+    }
+  }
+  return "custom";
+}
+
 function detectCalendarPreset(r: CalendarRights): RolePreset {
   for (const [name, preset] of Object.entries(CALENDAR_PRESETS) as [Exclude<RolePreset, "custom">, CalendarRights][]) {
     if ((Object.keys(preset) as (keyof CalendarRights)[]).every((k) => preset[k] === r[k])) {
@@ -87,12 +118,14 @@ function presetRights(kind: ShareKind, preset: RolePreset): AnyRights | undefine
   if (preset === "custom") return undefined;
   if (kind === "calendar") return CALENDAR_PRESETS[preset as keyof typeof CALENDAR_PRESETS];
   if (kind === "file") return FILE_PRESETS[preset as keyof typeof FILE_PRESETS];
+  if (kind === "mailbox") return MAILBOX_PRESETS[preset as keyof typeof MAILBOX_PRESETS];
   return ADDRESS_BOOK_PRESETS[preset as keyof typeof ADDRESS_BOOK_PRESETS];
 }
 
 function detectPreset(kind: ShareKind, rights: AnyRights): RolePreset {
   if (kind === "calendar") return detectCalendarPreset(rights as CalendarRights);
   if (kind === "file") return detectFilePreset(rights as FileNodeRights);
+  if (kind === "mailbox") return detectMailboxPreset(rights as MailboxRights);
   return detectAddressBookPreset(rights as AddressBookRights);
 }
 

lib/__tests__/jmap-client-extensions.test.ts

@@ -0,0 +1,225 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { JMAPClient } from '../jmap/client';
+
+/**
+ * Request shapes of the JMAP features layered on the core client: delta sync
+ * (Email/changes, Mailbox/changes), search snippets (SearchSnippet/get) and
+ * folder sharing (mail:share). Each asserts what goes over the wire and how
+ * the response is mapped back.
+ */
+
+function makeSession(accountCapabilities: Record<string, unknown> = {}) {
+  return {
+    capabilities: { 'urn:ietf:params:jmap:core': {} },
+    accounts: {
+      'acct-1': {
+        name: 'test',
+        isPersonal: true,
+        accountCapabilities: { 'urn:ietf:params:jmap:mail': {}, ...accountCapabilities },
+      },
+    },
+    primaryAccounts: { 'urn:ietf:params:jmap:mail': 'acct-1' },
+    apiUrl: 'https://mail.example.com/jmap/api',
+    downloadUrl: 'https://mail.example.com/jmap/download/{accountId}/{blobId}/{name}',
+    uploadUrl: 'https://mail.example.com/jmap/upload/{accountId}/',
+    eventSourceUrl: '',
+  };
+}
+
+function jsonResponse(body: unknown): Response {
+  return new Response(JSON.stringify(body), {
+    status: 200,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}
+
+type RequestBody = { using: string[]; methodCalls: [string, Record<string, unknown>, string][] };
+
+describe('JMAP client extensions', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+  let client: JMAPClient;
+  let requests: RequestBody[];
+
+  async function setup(accountCapabilities: Record<string, unknown> = {}) {
+    fetchSpy = vi.spyOn(globalThis, 'fetch');
+    fetchSpy.mockResolvedValueOnce(jsonResponse(makeSession(accountCapabilities)));
+    client = new JMAPClient('https://mail.example.com', 'user@test.com', 'pass123');
+    await client.connect();
+    fetchSpy.mockReset();
+    requests = [];
+  }
+
+  function answer(handler: (body: RequestBody) => unknown) {
+    fetchSpy.mockImplementation(async (_url: RequestInfo | URL, init?: RequestInit) => {
+      const body = JSON.parse(String(init?.body ?? '{}')) as RequestBody;
+      requests.push(body);
+      return jsonResponse(handler(body));
+    });
+  }
+
+  beforeEach(() => {
+    vi.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    client?.disconnect();
+    vi.restoreAllMocks();
+  });
+
+  describe('delta sync', () => {
+    it('maps Email/changes and passes sinceState / maxChanges', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [[
+          'Email/changes',
+          { accountId: 'acct-1', oldState: 's1', newState: 's2', hasMoreChanges: false, created: ['a'], updated: ['b'], destroyed: ['c'] },
+          '0',
+        ]],
+      }));
+
+      const delta = await client.getEmailChanges('s1', undefined, 200);
+
+      expect(requests[0].methodCalls[0]).toEqual(['Email/changes', { accountId: 'acct-1', sinceState: 's1', maxChanges: 200 }, '0']);
+      expect(delta).toEqual({
+        oldState: 's1', newState: 's2', hasMoreChanges: false,
+        created: ['a'], updated: ['b'], destroyed: ['c'], updatedProperties: null,
+      });
+    });
+
+    it('returns null when the server cannot calculate the delta', async () => {
+      await setup();
+      answer(() => ({ methodResponses: [['error', { type: 'cannotCalculateChanges' }, '0']] }));
+      expect(await client.getMailboxChanges('old')).toBeNull();
+    });
+
+    it('surfaces updatedProperties from Mailbox/changes', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [[
+          'Mailbox/changes',
+          { oldState: 'm1', newState: 'm2', hasMoreChanges: false, created: [], updated: ['inbox'], destroyed: [], updatedProperties: ['totalEmails', 'unreadEmails'] },
+          '0',
+        ]],
+      }));
+      const delta = await client.getMailboxChanges('m1');
+      expect(delta?.updatedProperties).toEqual(['totalEmails', 'unreadEmails']);
+    });
+
+    it('reports the Mailbox state per account from getAllMailboxesWithState', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [['Mailbox/get', { accountId: 'acct-1', state: 'm7', list: [{ id: 'inbox', name: 'Inbox', role: 'inbox' }] }, '0']],
+      }));
+      const { mailboxes, states } = await client.getAllMailboxesWithState();
+      expect(mailboxes.map((m) => m.id)).toEqual(['inbox']);
+      expect(states).toEqual({ 'acct-1': 'm7' });
+    });
+
+    it('reports the Email state a page was read at', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [
+          ['Email/query', { ids: ['e1'], total: 1 }, '0'],
+          ['Email/get', { state: 'e9', list: [{ id: 'e1', threadId: 't1', receivedAt: '2026-08-29T00:00:00Z', mailboxIds: { inbox: true }, keywords: {} }] }, '1'],
+        ],
+      }));
+      const page = await client.getEmails('inbox');
+      expect(page.state).toBe('e9');
+      expect(page.emails).toHaveLength(1);
+    });
+  });
+
+  describe('search snippets', () => {
+    it('chains SearchSnippet/get onto a text search and attaches the hits', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [
+          ['Email/query', { ids: ['e1', 'e2'], total: 2 }, '0'],
+          ['Email/get', { list: [
+            { id: 'e1', threadId: 't1', receivedAt: '2026-08-29T00:00:00Z', subject: 'Invoice', mailboxIds: {}, keywords: {} },
+            { id: 'e2', threadId: 't2', receivedAt: '2026-08-28T00:00:00Z', subject: 'Other', mailboxIds: {}, keywords: {} },
+          ] }, '1'],
+          ['SearchSnippet/get', { list: [
+            { emailId: 'e1', subject: '<mark>Invoice</mark>', preview: 'your <mark>invoice</mark> is attached' },
+            { emailId: 'e2', subject: null, preview: null },
+          ] }, 'snippets'],
+        ],
+      }));
+
+      const result = await client.searchEmails('invoice', 'inbox');
+
+      const snippetCall = requests[0].methodCalls[2];
+      expect(snippetCall[0]).toBe('SearchSnippet/get');
+      expect(snippetCall[1]).toEqual({
+        accountId: 'acct-1',
+        // The wildcard the search adds for prefix matching is stripped again.
+        filter: { operator: 'AND', conditions: [{ inMailbox: 'inbox' }, { text: 'invoice' }] },
+        '#emailIds': { resultOf: '0', name: 'Email/query', path: '/ids' },
+      });
+      expect(result.emails[0].searchSnippet).toEqual({ subject: '<mark>Invoice</mark>', preview: 'your <mark>invoice</mark> is attached' });
+      expect(result.emails[1].searchSnippet).toBeUndefined();
+    });
+
+    it('sends a no-op instead of SearchSnippet/get for a structural filter', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [
+          ['Email/query', { ids: [], total: 0 }, '0'],
+          ['Email/get', { list: [] }, '1'],
+          ['Core/echo', {}, 'snippets'],
+        ],
+      }));
+      await client.advancedSearchEmails({ operator: 'AND', conditions: [{ inMailbox: 'inbox' }, { hasKeyword: '$flagged' }] });
+      expect(requests[0].methodCalls[2][0]).toBe('Core/echo');
+    });
+
+    it('keeps the results when only the snippet call fails', async () => {
+      await setup();
+      answer(() => ({
+        methodResponses: [
+          ['Email/query', { ids: ['e1'], total: 1 }, '0'],
+          ['Email/get', { list: [{ id: 'e1', threadId: 't1', receivedAt: '2026-08-29T00:00:00Z', mailboxIds: {}, keywords: {} }] }, '1'],
+          ['error', { type: 'requestTooLarge' }, 'snippets'],
+        ],
+      }));
+      const result = await client.searchEmails('hello');
+      expect(result.emails).toHaveLength(1);
+      expect(result.emails[0].searchSnippet).toBeUndefined();
+    });
+  });
+
+  describe('mailbox sharing', () => {
+    it('is advertised only with the mail:share capability', async () => {
+      await setup();
+      expect(client.supportsMailboxSharing()).toBe(false);
+      client.disconnect();
+      vi.restoreAllMocks();
+      await setup({ 'urn:ietf:params:jmap:mail:share': {} });
+      expect(client.supportsMailboxSharing()).toBe(true);
+    });
+
+    it('reads shareWith with the extension in `using`', async () => {
+      await setup({ 'urn:ietf:params:jmap:mail:share': {} });
+      answer(() => ({
+        methodResponses: [['Mailbox/get', { list: [{ id: 'f1', shareWith: { p1: { mayReadItems: true } } }] }, '0']],
+      }));
+
+      const shareWith = await client.getMailboxShareWith('f1');
+
+      expect(requests[0].using).toContain('urn:ietf:params:jmap:mail:share');
+      expect(requests[0].methodCalls[0]).toEqual(['Mailbox/get', { accountId: 'acct-1', ids: ['f1'], properties: ['id', 'shareWith'] }, '0']);
+      expect(shareWith).toEqual({ p1: { mayReadItems: true } });
+    });
+
+    it('patches shareWith/{principal} on the owner account and checks the outcome', async () => {
+      await setup({ 'urn:ietf:params:jmap:mail:share': {} });
+      answer(() => ({ methodResponses: [['Mailbox/set', { updated: { f1: null } }, '0']] }));
+
+      await client.setMailboxShare('f1', 'p1', null, 'owner');
+      expect(requests[0].methodCalls[0]).toEqual(['Mailbox/set', { accountId: 'owner', update: { f1: { 'shareWith/p1': null } } }, '0']);
+
+      answer(() => ({ methodResponses: [['Mailbox/set', { notUpdated: { f1: { type: 'forbidden', description: 'no' } } }, '0']] }));
+      await expect(client.setMailboxShare('f1', 'p1', null)).rejects.toThrow('no');
+    });
+  });
+});

lib/jmap/client-interface.ts

@@ -1,4 +1,4 @@
-import type { Email, Mailbox, StateChange, AccountStates, CollectionChanges, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CreateCalendarOptions, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, FileNode, FileNodeRights, Principal, PushSubscription, EmailPushConfig, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
+import type { Email, Mailbox, MailboxRights, StateChange, AccountStates, CollectionChanges, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CreateCalendarOptions, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, FileNode, FileNodeRights, Principal, PushSubscription, EmailPushConfig, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
 import type { SieveScript, SieveCapabilities } from "./sieve-types";
 import type { SortLevel } from "@/lib/message-list-order";
 
@@ -126,6 +126,11 @@ export interface IJMAPClient {
   createMailbox(name: string, parentId?: string, accountId?: string): Promise<Mailbox>;
   updateMailbox(mailboxId: string, changes: { name?: string; parentId?: string | null; role?: string | null; sortOrder?: number }, accountId?: string): Promise<void>;
   deleteMailbox(mailboxId: string, accountId?: string): Promise<void>;
+  // Mailbox sharing (urn:ietf:params:jmap:mail:share). Optional: the demo
+  // client has no principals to share with.
+  supportsMailboxSharing?(accountId?: string): boolean;
+  getMailboxShareWith?(mailboxId: string, accountId?: string): Promise<Record<string, MailboxRights> | null>;
+  setMailboxShare?(mailboxId: string, principalId: string, rights: MailboxRights | null, accountId?: string): Promise<void>;
 
   // ── Emails ────────────────────────────────────────────────────
   // `pinnedFirst` sorts emails carrying the $pinned keyword to the top

lib/jmap/client.ts

@@ -1,5 +1,5 @@
 import { generateUUID } from '@/lib/utils';
-import type { Email, Mailbox, StateChange, AccountStates, CollectionChanges, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CalendarComponentType, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, CreateCalendarOptions, FileNode, FileNodeFilter, FileNodeRights, Principal, PushSubscription, EmailPushConfig, EmailSubmission, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
+import type { Email, Mailbox, MailboxRights, StateChange, AccountStates, CollectionChanges, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CalendarComponentType, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, CreateCalendarOptions, FileNode, FileNodeFilter, FileNodeRights, Principal, PushSubscription, EmailPushConfig, EmailSubmission, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
 import type { SieveScript, SieveCapabilities } from "./sieve-types";
 import type { IJMAPClient, KeywordDiscoveryResult, KeywordInfo, KeywordMigration } from "./client-interface";
 import { toWildcardQuery } from "./search-utils";
@@ -5127,6 +5127,73 @@ export class JMAPClient implements IJMAPClient {
     }
   }
 
+  // ── Mailbox sharing (urn:ietf:params:jmap:mail:share) ─────────────
+
+  static readonly MAIL_SHARE_CAPABILITY = 'urn:ietf:params:jmap:mail:share';
+
+  /** Whether the account's server accepts `shareWith` on Mailbox objects. */
+  supportsMailboxSharing(accountId?: string): boolean {
+    return this.hasAccountCapability(JMAPClient.MAIL_SHARE_CAPABILITY, accountId || this.accountId);
+  }
+
+  /**
+   * The current `shareWith` map of one folder. Fetched on demand (the share
+   * dialog) rather than with every folder list: Stalwart omits shareWith from
+   * Mailbox/get unless it is requested, and it is only needed here.
+   */
+  async getMailboxShareWith(mailboxId: string, accountId?: string): Promise<Record<string, MailboxRights> | null> {
+    const targetAccountId = accountId || this.accountId;
+    const response = await this.request([
+      ["Mailbox/get", {
+        accountId: targetAccountId,
+        ids: [mailboxId],
+        properties: ["id", "shareWith"],
+      }, "0"],
+    ], ["urn:ietf:params:jmap:core", "urn:ietf:params:jmap:mail", JMAPClient.MAIL_SHARE_CAPABILITY]);
+    const [method, result] = response.methodResponses?.[0] ?? [];
+    if (method !== "Mailbox/get") {
+      throw new Error((result as { description?: string })?.description || "Failed to load folder sharing");
+    }
+    const mailbox = ((result as { list?: Array<{ id: string; shareWith?: Record<string, MailboxRights> | null }> }).list ?? [])
+      .find((mb) => mb.id === mailboxId);
+    if (!mailbox) {
+      throw new Error("Folder not found");
+    }
+    return mailbox.shareWith ?? null;
+  }
+
+  /**
+   * Grants (`rights`) or revokes (`null`) a principal's access to a folder
+   * via a `shareWith/{principalId}` patch, like the calendar / address book
+   * variants.
+   */
+  async setMailboxShare(
+    mailboxId: string,
+    principalId: string,
+    rights: MailboxRights | null,
+    accountId?: string,
+  ): Promise<void> {
+    const targetAccountId = accountId || this.accountId;
+    const response = await this.request([
+      ["Mailbox/set", {
+        accountId: targetAccountId,
+        update: { [mailboxId]: { [`shareWith/${principalId}`]: rights } },
+      }, "0"],
+    ], ["urn:ietf:params:jmap:core", "urn:ietf:params:jmap:mail", JMAPClient.MAIL_SHARE_CAPABILITY]);
+
+    const [method, result] = response.methodResponses?.[0] ?? [];
+    if (method !== "Mailbox/set") {
+      throw new Error((result as { description?: string })?.description || "Failed to update folder share");
+    }
+    const setResult = result as { updated?: Record<string, unknown>; notUpdated?: Record<string, { description?: string }> };
+    if (setResult.notUpdated?.[mailboxId]) {
+      throw new Error(setResult.notUpdated[mailboxId].description || "Failed to update folder share");
+    }
+    if (!setResult.updated || !(mailboxId in setResult.updated)) {
+      throw new Error("Server did not confirm the share update");
+    }
+  }
+
   private async fetchPaginatedContacts(
     accountId: string,
     filter?: Record<string, unknown>,

lib/jmap/types.ts

@@ -210,24 +210,32 @@ export interface Mailbox {
   unreadEmails: number;
   totalThreads: number;
   unreadThreads: number;
-  myRights: {
-    mayReadItems: boolean;
-    mayAddItems: boolean;
-    mayRemoveItems: boolean;
-    maySetSeen: boolean;
-    maySetKeywords: boolean;
-    mayCreateChild: boolean;
-    mayRename: boolean;
-    mayDelete: boolean;
-    maySubmit: boolean;
-  };
+  myRights: MailboxRights;
   isSubscribed: boolean;
+  // Sharing (urn:ietf:params:jmap:mail:share): principalId -> rights. Only
+  // present when explicitly requested (see getMailboxShareWith).
+  shareWith?: Record<string, MailboxRights> | null;
   // Shared folder support
   accountId?: string;
   accountName?: string;
   isShared?: boolean;
 }
 
+// Mailbox rights (RFC 8621 §2 myRights; `mayShare` from the mail:share
+// extension). Also the value type of Mailbox.shareWith.
+export interface MailboxRights {
+  mayReadItems: boolean;
+  mayAddItems: boolean;
+  mayRemoveItems: boolean;
+  maySetSeen: boolean;
+  maySetKeywords: boolean;
+  mayCreateChild: boolean;
+  mayRename: boolean;
+  mayDelete: boolean;
+  maySubmit: boolean;
+  mayShare?: boolean;
+}
+
 export interface Thread {
   id: string;
   emailIds: string[];

locales/ar/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "مجلد فرعي جديد...",
     "new_folder": "مجلد جديد...",
     "rename": "إعادة تسمية...",
+    "share": "مشاركة...",
     "import_email": "استيراد ملف ‎.eml أو ‎.zip...",
     "empty_folder": "إفراغ المجلد",
     "empty_folder_generic": "إفراغ المجلد",

locales/ca/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Subcarpeta nova...",
     "new_folder": "Carpeta nova...",
     "rename": "Canvia el nom...",
+    "share": "Comparteix...",
     "import_email": "Importa .eml o .zip...",
     "empty_folder": "Buida la carpeta",
     "empty_folder_generic": "Buida la carpeta",

locales/cs/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nová podsložka...",
     "new_folder": "Nová složka...",
     "rename": "Přejmenovat...",
+    "share": "Sdílet...",
     "import_email": "Importovat .eml nebo .zip...",
     "empty_folder": "Vyprázdnit složku",
     "empty_folder_generic": "Vyprázdnit složku",

locales/da/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Ny undermappe...",
     "new_folder": "Ny mappe...",
     "rename": "Omdøb...",
+    "share": "Del...",
     "import_email": "Importér .eml eller .zip...",
     "empty_folder": "Tøm mappe",
     "empty_folder_generic": "Tøm mappe",

locales/de/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Neuer Unterordner...",
     "new_folder": "Neuer Ordner...",
     "rename": "Umbenennen...",
+    "share": "Freigeben...",
     "import_email": ".eml oder .zip importieren...",
     "empty_folder": "Ordner leeren",
     "empty_folder_generic": "Ordner leeren",

locales/en/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "New subfolder...",
     "new_folder": "New folder...",
     "rename": "Rename...",
+    "share": "Share...",
     "import_email": "Import .eml or .zip...",
     "empty_folder": "Empty folder",
     "empty_folder_generic": "Empty folder",

locales/es/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nueva subcarpeta...",
     "new_folder": "Nueva carpeta...",
     "rename": "Renombrar...",
+    "share": "Compartir...",
     "import_email": "Importar .eml o .zip...",
     "empty_folder": "Vaciar carpeta",
     "empty_folder_generic": "Vaciar carpeta",

locales/fa/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "زیرپوشه جدید...",
     "new_folder": "پوشه جدید...",
     "rename": "تغییر نام...",
+    "share": "اشتراک‌گذاری...",
     "import_email": "وارد کردن .eml/.zip...",
     "empty_folder": "خالی کردن پوشه",
     "empty_folder_generic": "خالی کردن پوشه",

locales/fr/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nouveau sous-dossier...",
     "new_folder": "Nouveau dossier...",
     "rename": "Renommer...",
+    "share": "Partager...",
     "import_email": "Importer .eml ou .zip...",
     "empty_folder": "Vider le dossier",
     "empty_folder_generic": "Vider le dossier",

locales/he/common.json

@@ -3418,6 +3418,7 @@
     "new_subfolder": "תת־תיקייה חדשה…",
     "new_folder": "תיקייה חדשה…",
     "rename": "שנה שם…",
+    "share": "שיתוף...",
     "import_email": "ייבא .eml או .zip…",
     "empty_folder": "תיקייה ריקה",
     "empty_folder_generic": "תיקייה ריקה",

locales/hu/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Új almappa...",
     "new_folder": "Új mappa...",
     "rename": "Átnevezés...",
+    "share": "Megosztás...",
     "import_email": "Importálás .eml vagy .zip fájlból...",
     "empty_folder": "Mappa ürítése",
     "empty_folder_generic": "Mappa ürítése",

locales/it/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nuova sottocartella...",
     "new_folder": "Nuova cartella...",
     "rename": "Rinomina...",
+    "share": "Condividi...",
     "import_email": "Importa .eml o .zip...",
     "empty_folder": "Svuota cartella",
     "empty_folder_generic": "Svuota cartella",

locales/ja/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "新しいサブフォルダー...",
     "new_folder": "新しいフォルダー...",
     "rename": "名前を変更...",
+    "share": "共有...",
     "import_email": ".eml または .zip をインポート...",
     "empty_folder": "フォルダーを空にする",
     "empty_folder_generic": "フォルダーを空にする",

locales/ko/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "새 하위 폴더...",
     "new_folder": "새 폴더...",
     "rename": "이름 바꾸기...",
+    "share": "공유...",
     "import_email": ".eml 또는 .zip 가져오기...",
     "empty_folder": "폴더 비우기",
     "empty_folder_generic": "폴더 비우기",

locales/lv/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Jauna apakšmape...",
     "new_folder": "Jauna mape...",
     "rename": "Pārsaukt...",
+    "share": "Kopīgot...",
     "import_email": "Importēt .eml vai .zip...",
     "empty_folder": "Iztukšot mapi",
     "empty_folder_generic": "Iztukšot mapi",

locales/mn/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Шинэ дэд хавтас...",
     "new_folder": "Шинэ хавтас...",
     "rename": "Нэр өөрчлөх...",
+    "share": "Хуваалцах...",
     "import_email": "Импорт .eml эсвэл .zip...",
     "empty_folder": "Хоосон хавтас",
     "empty_folder_generic": "Хоосон хавтас",

locales/nb/common.json

@@ -2205,6 +2205,7 @@
     "new_subfolder": "Ny undermappe...",
     "new_folder": "Ny mappe...",
     "rename": "Gi nytt navn...",
+    "share": "Del...",
     "import_email": "Importer .eml eller .zip...",
     "empty_folder": "Tøm mappe",
     "empty_folder_generic": "Tøm mappe",

locales/nl/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nieuwe submap...",
     "new_folder": "Nieuwe map...",
     "rename": "Hernoemen...",
+    "share": "Delen...",
     "import_email": ".eml of .zip importeren...",
     "empty_folder": "Map leegmaken",
     "empty_folder_generic": "Map leegmaken",

locales/pl/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nowy podfolder...",
     "new_folder": "Nowy folder...",
     "rename": "Zmień nazwę...",
+    "share": "Udostępnij...",
     "import_email": "Importuj .eml lub .zip...",
     "empty_folder": "Opróżnij folder",
     "empty_folder_generic": "Opróżnij folder",

locales/pt/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nova subpasta...",
     "new_folder": "Nova pasta...",
     "rename": "Renomear...",
+    "share": "Partilhar...",
     "import_email": "Importar .eml ou .zip...",
     "empty_folder": "Esvaziar pasta",
     "empty_folder_generic": "Esvaziar pasta",

locales/ro/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Subfolder nou...",
     "new_folder": "Dosar nou...",
     "rename": "Redenumire...",
+    "share": "Partajează...",
     "import_email": "Importați fișiere .eml sau .zip...",
     "empty_folder": "Folder gol",
     "empty_folder_generic": "Folder gol",

locales/ru/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Новая вложенная папка...",
     "new_folder": "Новая папка...",
     "rename": "Переименовать...",
+    "share": "Поделиться...",
     "import_email": "Импортировать .eml или .zip...",
     "empty_folder": "Очистить папку",
     "empty_folder_generic": "Очистить папку",

locales/sk/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Nový podpriečinok...",
     "new_folder": "Nový priečinok...",
     "rename": "Premenovať...",
+    "share": "Zdieľať...",
     "import_email": "Importovať .eml alebo .zip...",
     "empty_folder": "Vyprázdniť priečinok",
     "empty_folder_generic": "Vyprázdniť priečinok",

locales/tr/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Yeni alt klasör...",
     "new_folder": "Yeni klasör...",
     "rename": "Yeniden adlandır...",
+    "share": "Paylaş...",
     "import_email": ".eml veya .zip içe aktar...",
     "empty_folder": "Klasörü boşalt",
     "empty_folder_generic": "Klasörü boşalt",

locales/uk/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "Нова вкладена папка...",
     "new_folder": "Нова папка...",
     "rename": "Перейменувати...",
+    "share": "Поділитися...",
     "import_email": "Імпортувати .eml або .zip...",
     "empty_folder": "Очистити папку",
     "empty_folder_generic": "Очистити папку",

locales/zh-TW/common.json

@@ -2200,6 +2200,7 @@
     "new_subfolder": "新增子信件匣…",
     "new_folder": "新增信件匣…",
     "rename": "重新命名…",
+    "share": "共用...",
     "import_email": "匯入 .eml 或 .zip…",
     "empty_folder": "清空信件匣",
     "empty_folder_generic": "清空信件匣",

locales/zh/common.json

@@ -2207,6 +2207,7 @@
     "new_subfolder": "新建子文件夹...",
     "new_folder": "新建文件夹...",
     "rename": "重命名...",
+    "share": "共享...",
     "import_email": "导入 .eml 或 .zip...",
     "empty_folder": "清空文件夹",
     "empty_folder_generic": "清空文件夹",