diff --git a/components/mail/__tests__/mail-app-source-routing.test.tsx b/components/mail/__tests__/mail-app-source-routing.test.tsx
index 890635547..9348baa37 100644
--- a/components/mail/__tests__/mail-app-source-routing.test.tsx
+++ b/components/mail/__tests__/mail-app-source-routing.test.tsx
@@ -195,3 +195,63 @@ describe('the open message of a group account in a tag view', () => {
     expect(moveToMailbox).toHaveBeenCalledWith(client, 'eaaaaab', 'ce:b', 'login-a/ce:eaaaaab');
   });
 });
+
+describe('archiving a group message by month', () => {
+  // The group's Archive `k` already holds 2026/10, as the list names them:
+  // store ids and parents namespaced `ce:...`, the server's ids bare.
+  const year = { ...mailbox('y1', undefined, 'ce', true), name: '2026', parentId: 'ce:k' };
+  const month = { ...mailbox('m1', undefined, 'ce', true), name: '10', parentId: 'ce:y1' };
+  const withMonth = [...folders, year, month] as Mailbox[];
+
+  beforeEach(() => {
+    useSettingsStore.setState({ archiveMode: 'month' } as never);
+    client.createMailbox = vi.fn();
+  });
+
+  it('reuses the year and month folders in a tag view', async () => {
+    useEmailStore.setState({ mailboxes: withMonth } as never);
+    render(<MailApp />);
+    fireEvent.click(await screen.findByText('stub archive', {}, { timeout: 5000 }));
+    await waitFor(() => expect(moveThreadToMailbox).toHaveBeenCalled());
+    expect(client.createMailbox).not.toHaveBeenCalled();
+    expect(moveThreadToMailbox).toHaveBeenCalledWith(client, 'eaaaaab', 'ce:m1', 'login-a/ce:eaaaaab');
+  });
+
+  it('reuses them with the group\'s Inbox open', async () => {
+    const unstamped = { ...groupMessage, sourceClientAccountId: undefined, sourceAccountId: undefined } as Email;
+    useEmailStore.setState({ mailboxes: withMonth, selectedMailbox: 'ce:a', selectedKeyword: null, emails: [unstamped], selectedEmail: unstamped } as never);
+    render(<MailApp />);
+    fireEvent.click(await screen.findByText('stub archive', {}, { timeout: 5000 }));
+    await waitFor(() => expect(moveThreadToMailbox).toHaveBeenCalled());
+    expect(client.createMailbox).not.toHaveBeenCalled();
+    expect(moveThreadToMailbox).toHaveBeenCalledWith(client, 'eaaaaab', 'ce:m1', 'eaaaaab');
+  });
+
+  // The own account has a folder with the server id of the group's Archive,
+  // with 2026/10 below it (live on 0.16: a's "Shared" folder is `h`, and so
+  // was the group's Archive).
+  it('does not take the own account\'s namesake folders', async () => {
+    const unstamped = { ...groupMessage, sourceClientAccountId: undefined, sourceAccountId: undefined } as Email;
+    const ownNamesakes = [
+      { ...mailbox('k', undefined, 'cf'), name: 'Shared' },
+      { ...mailbox('oy', undefined, 'cf'), name: '2026', parentId: 'k' },
+      { ...mailbox('om', undefined, 'cf'), name: '10', parentId: 'oy' },
+    ];
+    useEmailStore.setState({ mailboxes: [...ownNamesakes, ...withMonth], selectedMailbox: 'ce:a', selectedKeyword: null, emails: [unstamped], selectedEmail: unstamped } as never);
+    render(<MailApp />);
+    fireEvent.click(await screen.findByText('stub archive', {}, { timeout: 5000 }));
+    await waitFor(() => expect(moveThreadToMailbox).toHaveBeenCalled());
+    expect(moveThreadToMailbox).toHaveBeenCalledWith(client, 'eaaaaab', 'ce:m1', 'eaaaaab');
+  });
+
+  it('creates a missing year folder in the group account with the group\'s Inbox open', async () => {
+    const unstamped = { ...groupMessage, sourceClientAccountId: undefined, sourceAccountId: undefined } as Email;
+    useSettingsStore.setState({ archiveMode: 'year' } as never);
+    (client.createMailbox as ReturnType<typeof vi.fn>).mockResolvedValue({ ...mailbox('y2', undefined, 'ce', true), id: 'y2', name: '2026' });
+    useEmailStore.setState({ selectedMailbox: 'ce:a', selectedKeyword: null, emails: [unstamped], selectedEmail: unstamped } as never);
+    render(<MailApp />);
+    fireEvent.click(await screen.findByText('stub archive', {}, { timeout: 5000 }));
+    await waitFor(() => expect(client.createMailbox).toHaveBeenCalled());
+    expect(client.createMailbox).toHaveBeenCalledWith('2026', 'k', 'ce', undefined);
+  });
+});
diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index c366a6cea..2a8d777be 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -2948,12 +2948,20 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
         const year = emailDate.getFullYear().toString();
         const month = (emailDate.getMonth() + 1).toString().padStart(2, '0');
         const archiveId = archiveMailbox.originalId || archiveMailbox.id;
+        // By the server's ids: the list names a shared account's folders and
+        // their parents `account:id`, so a group's year folder was never
+        // found again. The folders go in the Archive's own account, and only
+        // that account's are looked at: the own folders have server ids too.
+        const parentOf = (m: Mailbox) => (m.parentId && m.accountId && m.parentId.startsWith(`${m.accountId}:`)
+          ? m.parentId.slice(m.accountId.length + 1)
+          : m.parentId);
+        const folderAccountId = archiveMailbox.accountId ?? archiveAccountId;
+        const isFolder = (m: Mailbox, name: string, parentId: string) =>
+          m.name === name && parentOf(m) === parentId && (!folderAccountId || m.accountId === folderAccountId);
 
-        let yearMailbox = currentMailboxes.find(
-          m => m.name === year && m.parentId === archiveId
-        ) ?? null;
+        let yearMailbox = currentMailboxes.find(m => isFolder(m, year, archiveId)) ?? null;
         if (!yearMailbox) {
-          yearMailbox = await createMailboxWithHooks(archiveClient, year, archiveId, archiveAccountId);
+          yearMailbox = await createMailboxWithHooks(archiveClient, year, archiveId, folderAccountId);
           if (!yearMailbox) return; // a plugin refused the folder
           await refreshMailboxes();
         }
@@ -2963,11 +2971,9 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
         } else {
           const yearId = yearMailbox.originalId || yearMailbox.id;
           const afterYear = readMailboxes();
-          let monthMailbox = afterYear.find(
-            m => m.name === month && m.parentId === yearId
-          ) ?? null;
+          let monthMailbox = afterYear.find(m => isFolder(m, month, yearId)) ?? null;
           if (!monthMailbox) {
-            monthMailbox = await createMailboxWithHooks(archiveClient, month, yearId, archiveAccountId);
+            monthMailbox = await createMailboxWithHooks(archiveClient, month, yearId, folderAccountId);
             if (!monthMailbox) return; // a plugin refused the folder
             await refreshMailboxes();
           }