feat: add auto-select reply identity feature with settings and localization

app/[locale]/page.tsx

@@ -1539,6 +1539,7 @@ export default function Home() {
                     from: selectedEmail.from,
                     to: selectedEmail.to,
                     cc: selectedEmail.cc,
+                    bcc: selectedEmail.bcc,
                     subject: selectedEmail.subject,
                     body: selectedEmail.bodyValues?.[selectedEmail.textBody?.[0]?.partId || '']?.value || selectedEmail.preview || '',
                     htmlBody: selectedEmail.bodyValues?.[selectedEmail.htmlBody?.[0]?.partId || '']?.value || undefined,

app/admin/policy/page.tsx

@@ -32,6 +32,7 @@ const RESTRICTABLE_SETTINGS = [
   { key: 'externalContentPolicy', label: 'External Content Policy', category: 'Email', type: 'enum', allowedValues: ['allow', 'block', 'ask'] },
   { key: 'sendConfirmation', label: 'Send Confirmation', category: 'Composer', type: 'boolean' },
   { key: 'defaultReplyMode', label: 'Default Reply Mode', category: 'Composer', type: 'enum', allowedValues: ['reply', 'reply-all'] },
+  { key: 'autoSelectReplyIdentity', label: 'Auto-select Reply Identity', category: 'Composer', type: 'boolean' },
   { key: 'plainTextMode', label: 'Plain Text Only', category: 'Composer', type: 'boolean' },
   { key: 'sessionTimeout', label: 'Session Timeout', category: 'Privacy', type: 'number' },
   { key: 'emailNotificationsEnabled', label: 'Email Notifications', category: 'Notifications', type: 'boolean' },

components/email/email-composer.tsx

@@ -29,6 +29,7 @@ import { TemplatePicker } from "@/components/templates/template-picker";
 import { TemplateForm } from "@/components/templates/template-form";
 import type { EmailTemplate } from "@/lib/template-types";
 import { appendPlainTextSignature, getPlainTextSignature } from "@/lib/signature-utils";
+import { findReplyIdentityId } from "@/lib/reply-identity";
 import { RichTextEditor } from "@/components/email/rich-text-editor";
 
 /** Strip HTML tags and decode entities to get a plain-text version */
@@ -78,6 +79,7 @@ interface EmailComposerProps {
     from?: { email?: string; name?: string }[];
     to?: { email?: string; name?: string }[];
     cc?: { email?: string; name?: string }[];
+    bcc?: { email?: string; name?: string }[];
     subject?: string;
     body?: string;
     htmlBody?: string;
@@ -100,6 +102,7 @@ export function EmailComposer({
   const tCommon = useTranslations('common');
   const timeFormat = useSettingsStore((state) => state.timeFormat);
   const plainTextMode = useSettingsStore((state) => state.plainTextMode);
+  const autoSelectReplyIdentity = useSettingsStore((state) => state.autoSelectReplyIdentity);
 
   // Initialize with reply/forward data if provided
   const getInitialTo = () => {
@@ -226,6 +229,31 @@ export function EmailComposer({
   const currentIdentity = selectedIdentityId
     ? identities.find((identity) => identity.id === selectedIdentityId) || primaryIdentity
     : primaryIdentity;
+  useEffect(() => {
+    if (!autoSelectReplyIdentity) return;
+    if (selectedIdentityId || initialData?.selectedIdentityId) return;
+    if (mode !== 'reply' && mode !== 'replyAll') return;
+
+    const matchedIdentityId = findReplyIdentityId(identities, {
+      to: replyTo?.to,
+      cc: replyTo?.cc,
+      bcc: replyTo?.bcc,
+    });
+
+    if (matchedIdentityId) {
+      setSelectedIdentityId(matchedIdentityId);
+    }
+  }, [
+    autoSelectReplyIdentity,
+    identities,
+    initialData?.selectedIdentityId,
+    mode,
+    replyTo?.bcc,
+    replyTo?.cc,
+    replyTo?.to,
+    selectedIdentityId,
+  ]);
+
   const composerSignatureHtml = currentIdentity?.htmlSignature
     ? `<div>${sanitizeEmailHtml(currentIdentity.htmlSignature)}</div>`
     : currentIdentity?.textSignature

components/settings/email-settings.tsx

@@ -40,6 +40,7 @@ export function EmailSettings() {
     permanentlyDeleteJunk,
     showPreview,
     disableThreading,
+    autoSelectReplyIdentity,
     plainTextMode,
     emailsPerPage,
     externalContentPolicy,
@@ -230,6 +231,13 @@ export function EmailSettings() {
         />
       </SettingItem>
 
+      <SettingItem label={t('auto_select_reply_identity.label')} description={t('auto_select_reply_identity.description')}>
+        <ToggleSwitch
+          checked={autoSelectReplyIdentity}
+          onChange={(checked) => updateSetting('autoSelectReplyIdentity', checked)}
+        />
+      </SettingItem>
+
       {/* Quick Hover Actions */}
       {isFeatureEnabled('hoverActionsConfigEnabled') && (
       <div className="py-3 border-b border-border space-y-3">

lib/__tests__/reply-identity.test.ts

@@ -0,0 +1,52 @@
+import { describe, expect, it } from 'vitest';
+import { findReplyIdentityId } from '../reply-identity';
+import type { Identity } from '../jmap/types';
+
+const identities: Identity[] = [
+  {
+    id: 'primary',
+    name: 'Harry Primary',
+    email: 'harry@primary.com',
+    mayDelete: false,
+  },
+  {
+    id: 'secondary',
+    name: 'Harry Secondary',
+    email: 'harry@secondary.com',
+    mayDelete: false,
+  },
+];
+
+describe('findReplyIdentityId', () => {
+  it('matches the identity that received the original message', () => {
+    const selected = findReplyIdentityId(identities, {
+      to: [{ email: 'harry@secondary.com' }],
+    });
+
+    expect(selected).toBe('secondary');
+  });
+
+  it('matches case-insensitively across recipients', () => {
+    const selected = findReplyIdentityId(identities, {
+      cc: [{ email: 'HARRY@PRIMARY.COM' }],
+    });
+
+    expect(selected).toBe('primary');
+  });
+
+  it('falls back to sub-address matching when needed', () => {
+    const selected = findReplyIdentityId(identities, {
+      to: [{ email: 'harry+news@secondary.com' }],
+    });
+
+    expect(selected).toBe('secondary');
+  });
+
+  it('returns null when no reply recipient matches an identity', () => {
+    const selected = findReplyIdentityId(identities, {
+      to: [{ email: 'other@example.com' }],
+    });
+
+    expect(selected).toBeNull();
+  });
+});

lib/reply-identity.ts

@@ -0,0 +1,62 @@
+import type { Identity } from '@/lib/jmap/types';
+
+interface ReplyRecipient {
+  email?: string | null;
+}
+
+interface ReplyRecipients {
+  to?: ReplyRecipient[];
+  cc?: ReplyRecipient[];
+  bcc?: ReplyRecipient[];
+}
+
+function normalizeEmailAddress(email: string): string {
+  return email.trim().toLowerCase();
+}
+
+function normalizeBaseEmailAddress(email: string): string {
+  const normalized = normalizeEmailAddress(email);
+  const atIndex = normalized.indexOf('@');
+
+  if (atIndex <= 0) {
+    return normalized;
+  }
+
+  const localPart = normalized.slice(0, atIndex);
+  const domain = normalized.slice(atIndex + 1);
+  const plusIndex = localPart.indexOf('+');
+
+  return `${plusIndex >= 0 ? localPart.slice(0, plusIndex) : localPart}@${domain}`;
+}
+
+export function findReplyIdentityId(
+  identities: Identity[],
+  recipients?: ReplyRecipients,
+): string | null {
+  if (identities.length === 0 || !recipients) {
+    return null;
+  }
+
+  const receivedAddresses = [
+    ...(recipients.to || []),
+    ...(recipients.cc || []),
+    ...(recipients.bcc || []),
+  ]
+    .map((recipient) => recipient.email?.trim())
+    .filter((email): email is string => Boolean(email));
+
+  if (receivedAddresses.length === 0) {
+    return null;
+  }
+
+  const exactMatches = new Set(receivedAddresses.map(normalizeEmailAddress));
+  const exactIdentity = identities.find((identity) => exactMatches.has(normalizeEmailAddress(identity.email)));
+  if (exactIdentity) {
+    return exactIdentity.id;
+  }
+
+  const baseMatches = new Set(receivedAddresses.map(normalizeBaseEmailAddress));
+  const baseIdentity = identities.find((identity) => baseMatches.has(normalizeBaseEmailAddress(identity.email)));
+
+  return baseIdentity?.id ?? null;
+}

locales/de/common.json

@@ -811,6 +811,10 @@
         "label": "Nur Klartext",
         "description": "Rich-Text-Editor deaktivieren und alle E-Mails nur als Klartext senden, einschließlich Antworten und Weiterleitungen"
       },
+      "auto_select_reply_identity": {
+        "label": "Antwortadresse automatisch wählen",
+        "description": "Beim Antworten die Absenderadresse automatisch auf die Identität umstellen, die die ursprüngliche Nachricht erhalten hat"
+      },
       "attachment_click_action": {
         "label": "Aktion beim Klick auf Anhänge",
         "description": "Festlegen, ob ein Dateianhang beim Anklicken in der Vorschau geöffnet oder sofort heruntergeladen wird",

locales/en/common.json

@@ -811,6 +811,10 @@
         "label": "Plain Text Only",
         "description": "Disable the rich text editor and send all emails as plain text only, including replies and forwards"
       },
+      "auto_select_reply_identity": {
+        "label": "Auto-select Reply Address",
+        "description": "When replying, automatically switch the From address to the identity that originally received the message"
+      },
       "attachment_click_action": {
         "label": "Attachment Click Action",
         "description": "Choose whether clicking a file attachment previews it or downloads it immediately",

locales/es/common.json

@@ -807,6 +807,10 @@
         "label": "Solo texto sin formato",
         "description": "Desactivar el editor de texto enriquecido y enviar todos los correos solo como texto sin formato, incluyendo respuestas y reenvíos"
       },
+      "auto_select_reply_identity": {
+        "label": "Seleccionar dirección de respuesta automáticamente",
+        "description": "Al responder, cambia automáticamente la dirección del remitente a la identidad que recibió el mensaje original"
+      },
       "show_preview": {
         "label": "Mostrar Vista Previa",
         "description": "Mostrar vista previa del correo en la lista"

locales/fr/common.json

@@ -807,6 +807,10 @@
         "label": "Texte brut uniquement",
         "description": "Désactiver l'éditeur de texte enrichi et envoyer tous les e-mails en texte brut uniquement, y compris les réponses et les transferts"
       },
+      "auto_select_reply_identity": {
+        "label": "Sélection automatique de l'adresse de réponse",
+        "description": "Lors d'une réponse, bascule automatiquement l'adresse d'expédition vers l'identité qui a reçu le message d'origine"
+      },
       "show_preview": {
         "label": "Afficher l'aperçu",
         "description": "Afficher l'aperçu de l'email dans la liste"

locales/it/common.json

@@ -807,6 +807,10 @@
         "label": "Solo testo normale",
         "description": "Disabilita l'editor di testo formattato e invia tutte le email solo come testo normale, incluse risposte e inoltri"
       },
+      "auto_select_reply_identity": {
+        "label": "Seleziona automaticamente l'indirizzo di risposta",
+        "description": "Quando rispondi, passa automaticamente l'indirizzo mittente all'identità che ha ricevuto il messaggio originale"
+      },
       "show_preview": {
         "label": "Mostra anteprima testo",
         "description": "Visualizza l'anteprima del messaggio nell'elenco"

locales/ja/common.json

@@ -807,6 +807,10 @@
         "label": "プレーンテキストのみ",
         "description": "リッチテキストエディターを無効にし、返信や転送を含むすべてのメールをプレーンテキストのみで送信します"
       },
+      "auto_select_reply_identity": {
+        "label": "返信元アドレスを自動選択",
+        "description": "返信時に、元のメッセージを受信したIDへ差出人アドレスを自動的に切り替えます"
+      },
       "show_preview": {
         "label": "プレビューテキストを表示",
         "description": "リストにメールのプレビューを表示"

locales/nl/common.json

@@ -807,6 +807,10 @@
         "label": "Alleen platte tekst",
         "description": "Schakel de rich text-editor uit en verzend alle e-mails alleen als platte tekst, inclusief antwoorden en doorgestuurde berichten"
       },
+      "auto_select_reply_identity": {
+        "label": "Antwoordadres automatisch selecteren",
+        "description": "Schakel bij het beantwoorden automatisch het Van-adres om naar de identiteit die het oorspronkelijke bericht ontving"
+      },
       "show_preview": {
         "label": "Voorbeeldtekst tonen",
         "description": "E-mailvoorbeeld weergeven in de lijst"

locales/pt/common.json

@@ -807,6 +807,10 @@
         "label": "Apenas texto simples",
         "description": "Desativar o editor de texto formatado e enviar todos os e-mails apenas como texto simples, incluindo respostas e encaminhamentos"
       },
+      "auto_select_reply_identity": {
+        "label": "Selecionar automaticamente o endereço de resposta",
+        "description": "Ao responder, muda automaticamente o endereço do remetente para a identidade que recebeu a mensagem original"
+      },
       "show_preview": {
         "label": "Mostrar Texto de Visualização",
         "description": "Exibir visualização do e-mail na lista"

locales/ru/common.json

@@ -807,6 +807,10 @@
         "label": "Только простой текст",
         "description": "Отключить редактор форматированного текста и отправлять все письма только в виде простого текста, включая ответы и пересылки"
       },
+      "auto_select_reply_identity": {
+        "label": "Автоматически выбирать адрес для ответа",
+        "description": "При ответе автоматически переключать адрес отправителя на ту учетную запись, которая получила исходное сообщение"
+      },
       "show_preview": {
         "label": "Показывать текст предпросмотра",
         "description": "Отображать предпросмотр письма в списке"

stores/__tests__/settings-store-attachments.test.ts

@@ -29,4 +29,14 @@ describe('settings-store attachment action', () => {
 
     expect(exported.calendarInvitationParsingEnabled).toBe(false);
   });
+
+  it('includes reply identity auto-selection in exported settings', () => {
+    useSettingsStore.getState().updateSetting('autoSelectReplyIdentity', true);
+
+    const exported = JSON.parse(useSettingsStore.getState().exportSettings()) as {
+      autoSelectReplyIdentity?: boolean;
+    };
+
+    expect(exported.autoSelectReplyIdentity).toBe(true);
+  });
 });

stores/settings-store.ts

@@ -119,6 +119,7 @@ interface SettingsState {
   autoSaveDraftInterval: number; // milliseconds
   sendConfirmation: boolean;
   defaultReplyMode: ReplyMode;
+  autoSelectReplyIdentity: boolean;
   plainTextMode: boolean; // Send plain text only (no rich text editor)
 
   // Privacy & Security
@@ -238,6 +239,7 @@ const DEFAULT_SETTINGS = {
   autoSaveDraftInterval: 60000, // 1 minute
   sendConfirmation: false,
   defaultReplyMode: 'reply' as ReplyMode,
+  autoSelectReplyIdentity: false,
   plainTextMode: false,
 
   // Privacy & Security
@@ -346,6 +348,7 @@ export const useSettingsStore = create<SettingsState>()(
           autoSaveDraftInterval: state.autoSaveDraftInterval,
           sendConfirmation: state.sendConfirmation,
           defaultReplyMode: state.defaultReplyMode,
+          autoSelectReplyIdentity: state.autoSelectReplyIdentity,
           plainTextMode: state.plainTextMode,
           sessionTimeout: state.sessionTimeout,
           emailNotificationsEnabled: state.emailNotificationsEnabled,