diff --git a/stores/__tests__/settings-store-preferred-identity.test.ts b/stores/__tests__/settings-store-preferred-identity.test.ts
index fac032a54..2f354e6ef 100644
--- a/stores/__tests__/settings-store-preferred-identity.test.ts
+++ b/stores/__tests__/settings-store-preferred-identity.test.ts
@@ -56,6 +56,54 @@ describe('settings-store per-account preferredIdentityIds (issue #507)', () => {
     });
   });
 
+  // Regression: multi-account logins used to clobber the per-account map. Each
+  // account's server load must merge only its OWN entry, preserving the others,
+  // so the composer's default sender no longer depends on login order.
+  describe('importSettings per-account merge (multi-account login order)', () => {
+    beforeEach(() => useSettingsStore.setState({ preferredIdentityIds: {}, allMailFolderIds: {} }));
+
+    it('a per-account server load updates only its own entry, preserving others', () => {
+      useSettingsStore.setState({ preferredIdentityIds: { 'a@h': 'idA', 'b@h': 'idB' } });
+      // Account B's server blob carries a STALE entry for A plus B's own value.
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ preferredIdentityIds: { 'a@h': 'STALE', 'b@h': 'idB2' } }),
+        { serverAccountId: 'b@h' },
+      );
+      const map = useSettingsStore.getState().preferredIdentityIds;
+      expect(map['a@h']).toBe('idA');  // preserved, not clobbered by B's stale copy
+      expect(map['b@h']).toBe('idB2'); // B is authoritative for itself
+    });
+
+    it('fills the loading account entry when the map had none, without importing others', () => {
+      useSettingsStore.setState({ preferredIdentityIds: { 'a@h': 'idA' } });
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ preferredIdentityIds: { 'a@h': 'x', 'b@h': 'idB' } }),
+        { serverAccountId: 'b@h' },
+      );
+      // Only b@h (the loading account) is trusted; a@h keeps its local value.
+      expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'a@h': 'idA', 'b@h': 'idB' });
+    });
+
+    it('a file import (no serverAccountId) still replaces the whole map', () => {
+      useSettingsStore.setState({ preferredIdentityIds: { 'a@h': 'idA' } });
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ preferredIdentityIds: { 'b@h': 'idB' } }),
+      );
+      expect(useSettingsStore.getState().preferredIdentityIds).toEqual({ 'b@h': 'idB' });
+    });
+
+    it('applies the same per-account merge to allMailFolderIds', () => {
+      useSettingsStore.setState({ allMailFolderIds: { 'a@h': ['x'], 'b@h': ['y'] } });
+      useSettingsStore.getState().importSettings(
+        JSON.stringify({ allMailFolderIds: { 'a@h': ['STALE'], 'b@h': ['y2'] } }),
+        { serverAccountId: 'b@h' },
+      );
+      const map = useSettingsStore.getState().allMailFolderIds;
+      expect(map['a@h']).toEqual(['x']);
+      expect(map['b@h']).toEqual(['y2']);
+    });
+  });
+
   describe('migrateSettings identity map', () => {
     it('adds an empty preferredIdentityIds map for pre-v6 users', () => {
       const out = migrateSettings({ allMailFolderIds: {} }, 6) as unknown as Record<string, unknown>;
diff --git a/stores/settings-store.ts b/stores/settings-store.ts
index f82ab866c..033688492 100644
--- a/stores/settings-store.ts
+++ b/stores/settings-store.ts
@@ -4,6 +4,7 @@ import { useThemeStore } from './theme-store';
 import { useLocaleStore } from './locale-store';
 import type { NotificationSoundChoice } from '@/lib/notification-sound';
 import { apiFetch } from '@/lib/browser-navigation';
+import { generateAccountId } from '@/lib/account-utils';
 import {
   DEFAULT_SUB_ADDRESS_DELIMITER,
   isValidSubAddressDelimiter,
@@ -418,7 +419,7 @@ interface SettingsState {
   ) => void;
   resetToDefaults: () => void;
   exportSettings: () => string;
-  importSettings: (json: string) => boolean;
+  importSettings: (json: string, opts?: { serverAccountId?: string }) => boolean;
 
   // Folder icons
   setFolderIcon: (mailboxId: string, icon: string) => void;
@@ -772,7 +773,7 @@ export const useSettingsStore = create<SettingsState>()(
         return JSON.stringify(settings, null, 2);
       },
 
-      importSettings: (json: string) => {
+      importSettings: (json: string, opts?: { serverAccountId?: string }) => {
         try {
           const settings = JSON.parse(json);
 
@@ -808,6 +809,24 @@ export const useSettingsStore = create<SettingsState>()(
               if (DEVICE_LOCAL_SETTING_KEYS.has(key)) {
                 return;
               }
+              // Per-account maps (accountId -> value) live in every account's
+              // synced settings blob, so a per-account server load must NOT
+              // replace the whole map - each account's server is authoritative
+              // only for its OWN entry. Merging preserves the other accounts'
+              // local values; without this the last account to load clobbers
+              // the map and the composer picks another account's default sender
+              // (login-order dependent). File imports (no serverAccountId)
+              // still replace wholesale.
+              if (opts?.serverAccountId && (key === 'preferredIdentityIds' || key === 'allMailFolderIds')) {
+                const existing = (get() as unknown as Record<string, unknown>)[key];
+                const merged: Record<string, unknown> = isPlainRecord(existing) ? { ...existing } : {};
+                const incoming = settings[key] as Record<string, unknown>;
+                if (Object.prototype.hasOwnProperty.call(incoming, opts.serverAccountId)) {
+                  merged[opts.serverAccountId] = incoming[opts.serverAccountId];
+                }
+                set({ [key]: merged });
+                return;
+              }
               set({ [key]: settings[key] });
             }
           });
@@ -980,7 +999,11 @@ export const useSettingsStore = create<SettingsState>()(
           }
           if (settings && typeof settings === 'object') {
             isLoadingFromServer = true;
-            get().importSettings(JSON.stringify(settings));
+            // Merge (not replace) per-account maps for the account being loaded,
+            // so multi-account logins don't clobber each other by login order.
+            get().importSettings(JSON.stringify(settings), {
+              serverAccountId: generateAccountId(username, serverUrl),
+            });
             isLoadingFromServer = false;
             syncLog('Settings loaded from server successfully');
             // The per-account preferred sender identity (#507) is re-applied by