diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index 9c69bd96e..d24d4e61d 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -3172,6 +3172,8 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
         attachments: [payload.attachment],
         headers: labelSourceHeaders(email.headers),
         originalFrom: email.from,
+        // The login holding the message, whose blob this attachment is.
+        accountId: email.sourceClientAccountId ?? viewingAccountId ?? undefined,
       },
     });
     setComposerMode('forward');
diff --git a/components/pro/__tests__/pro-email-reply-account.test.tsx b/components/pro/__tests__/pro-email-reply-account.test.tsx
new file mode 100644
index 000000000..d0d514717
--- /dev/null
+++ b/components/pro/__tests__/pro-email-reply-account.test.tsx
@@ -0,0 +1,137 @@
+import { render, waitFor } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import React from 'react';
+
+/**
+ * A reply, forward or forward-as-attachment from a Pro message view names the
+ * login holding the message. The composer defaults its From to that account and
+ * copies the original's parts from it; without the login it takes the message
+ * for the active account's, so the reply goes out from the wrong account, or
+ * fails on a blobId that account doesn't have (#1511).
+ */
+
+const viewerProps = vi.hoisted(() => ({ current: null as Record<string, unknown> | null }));
+
+vi.mock('@/components/email/email-viewer', () => ({
+  EmailViewer: (props: Record<string, unknown>) => {
+    viewerProps.current = props;
+    return null;
+  },
+}));
+vi.mock('@/components/error', () => ({
+  ErrorBoundary: ({ children }: { children: React.ReactNode }) => React.createElement(React.Fragment, null, children),
+  EmailViewerErrorFallback: () => null,
+}));
+vi.mock('next-intl', () => ({ useTranslations: () => (key: string) => key }));
+vi.mock('@/stores/toast-store', () => ({ toast: { error: () => {}, success: () => {} } }));
+vi.mock('@/hooks/use-pro-multi-account-identities', async (importOriginal) => ({
+  ...await importOriginal<typeof import('@/hooks/use-pro-multi-account-identities')>(),
+  useProMultiAccountIdentities: () => ({ enabled: true, groups: [], allIdentities: [] }),
+}));
+
+const { openComposeTab } = vi.hoisted(() => ({ openComposeTab: vi.fn() }));
+
+function selectorStore<T extends object>(state: T) {
+  const hook = (sel?: (s: T) => unknown) => (typeof sel === 'function' ? sel(state) : state);
+  hook.getState = () => state;
+  return hook;
+}
+
+const auth = vi.hoisted(() => ({
+  state: { client: null as unknown, activeAccountId: 'login-a', getClientForAccount: () => undefined },
+}));
+vi.mock('@/stores/auth-store', () => ({ useAuthStore: selectorStore(auth.state) }));
+
+const MESSAGE = vi.hoisted(() => ({
+  id: 'eaaaaab',
+  blobId: 'blob-of-message',
+  threadId: 't1',
+  mailboxIds: { inbox: true },
+  keywords: { $seen: true },
+  from: [{ email: 'sender@example.com', name: 'Sender' }],
+  to: [{ email: 'b@example.org' }],
+  subject: 'Hello',
+  receivedAt: '2026-01-01T00:00:00Z',
+  bodyValues: { '1': { value: '<img src="cid:logo">' } },
+  textBody: [],
+  htmlBody: [{ partId: '1', type: 'text/html' }],
+  attachments: [{ blobId: 'blob-of-logo', name: 'logo.png', type: 'image/png', size: 10, cid: 'logo', disposition: 'inline' }],
+}));
+
+vi.mock('@/stores/email-store', () => ({
+  resolveEmailBlobContext: (_email: unknown, client: unknown) => ({ client, clientAccountId: auth.state.activeAccountId }),
+  useEmailStore: selectorStore({
+    fetchEmailContent: async () => MESSAGE,
+    deleteEmail: async () => {},
+    markAsRead: async () => {},
+    toggleStar: async () => {},
+    moveToMailbox: async () => {},
+    setEmailKeywordsLocal: () => {},
+    mailboxes: [],
+    emails: [],
+    selectedEmail: null,
+  }),
+}));
+vi.mock('@/stores/identity-store', () => ({ useIdentityStore: selectorStore({ identities: [] }) }));
+vi.mock('@/stores/settings-store', () => ({ useSettingsStore: selectorStore({ emailDownloadTemplate: '' }) }));
+vi.mock('@/stores/pro-tab-store', () => ({
+  useProTabStore: selectorStore({ openComposeTab, closeTab: () => {}, updateTabTitle: () => {} }),
+}));
+
+import { ProEmailView } from '../pro-email-tab-body';
+
+const ACTIONS = ['onReply', 'onReplyAll', 'onForward', 'onForwardAsAttachment'] as const;
+
+async function open(props: { clientAccountId?: string; accountId?: string; override?: boolean }) {
+  const client = { getEmail: vi.fn().mockResolvedValue(MESSAGE), getUsername: () => 'b@example.org', getAccountId: () => 'jmap-b' };
+  const { override = true, ...rest } = props;
+  if (!override) auth.state.client = client;
+  const view = render(<ProEmailView emailId="eaaaaab" client={override ? client as never : undefined} {...rest} onClose={vi.fn()} />);
+  await waitFor(() => expect(viewerProps.current?.email).toBeTruthy());
+  return { ...view, client };
+}
+
+function composeFrom(action: typeof ACTIONS[number]) {
+  openComposeTab.mockClear();
+  (viewerProps.current![action] as () => void)();
+  expect(openComposeTab).toHaveBeenCalledTimes(1);
+  return openComposeTab.mock.calls[0][0].replyTo as Record<string, unknown>;
+}
+
+describe('compose actions of a Pro message view', () => {
+  beforeEach(() => {
+    viewerProps.current = null;
+    openComposeTab.mockClear();
+    auth.state.activeAccountId = 'login-a';
+    auth.state.client = null;
+  });
+
+  it.each(ACTIONS)('%s of another login names that login', async (action) => {
+    await open({ clientAccountId: 'login-b' });
+
+    expect(composeFrom(action).accountId).toBe('login-b');
+  });
+
+  it.each(ACTIONS)('%s of a shared folder of another login names the login, not the owner', async (action) => {
+    await open({ clientAccountId: 'login-b', accountId: 'jmap-team' });
+
+    expect(composeFrom(action).accountId).toBe('login-b');
+  });
+
+  it.each(ACTIONS)('%s of the active login names the active login', async (action) => {
+    await open({ override: false });
+
+    expect(composeFrom(action).accountId).toBe('login-a');
+  });
+
+  it('keeps naming the login the message was read in after the active account changes', async () => {
+    // A message of b isn't foreign while b is active, so it carries no stamp.
+    auth.state.activeAccountId = 'login-b';
+    const { rerender, client } = await open({ clientAccountId: 'login-b' });
+
+    auth.state.activeAccountId = 'login-a';
+    rerender(<ProEmailView emailId="eaaaaab" client={client as never} clientAccountId="login-b" onClose={vi.fn()} />);
+
+    expect(composeFrom('onReply').accountId).toBe('login-b');
+  });
+});
diff --git a/components/pro/pro-email-tab-body.tsx b/components/pro/pro-email-tab-body.tsx
index 024742440..bc8d0fbfc 100644
--- a/components/pro/pro-email-tab-body.tsx
+++ b/components/pro/pro-email-tab-body.tsx
@@ -111,7 +111,17 @@ export function stampProSource(email: Email, source: EmailBlobContext, loginId:
   return { ...email, sourceClientAccountId: login, sourceAccountId: account };
 }
 
-function buildReplyContext(email: Email): ProReplyContext {
+/**
+ * The login holding the message, for ProReplyContext.accountId: the composer
+ * defaults its From to that account and copies the original's parts from it
+ * (#1104, #1511). email.accountId can't say: it is for display, and a message
+ * read through client.getEmail() has none.
+ */
+function replyLogin(email: Email, source: EmailWriteSource): string | undefined {
+  return email.sourceClientAccountId ?? source.clientAccountId;
+}
+
+function buildReplyContext(email: Email, source: EmailWriteSource): ProReplyContext {
   return {
     from: email.from,
     replyToAddresses: email.replyTo,
@@ -122,7 +132,7 @@ function buildReplyContext(email: Email): ProReplyContext {
     subject: email.subject,
     ...getQuoteBodies(email),
     receivedAt: email.receivedAt,
-    accountId: email.accountId,
+    accountId: replyLogin(email, source),
     attachments: email.attachments,
     messageId: email.messageId,
     inReplyTo: email.inReplyTo,
@@ -285,7 +295,7 @@ export function ProEmailView({ emailId, client: clientOverride, clientAccountId,
     if (!email) return;
     openComposeTab({
       mode: 'reply',
-      replyTo: buildReplyContext(email),
+      replyTo: buildReplyContext(email, writeSource),
       sourceEmailId: email.id,
       sourceEmailSource: writeSource,
       initialDraftText: draftText,
@@ -305,7 +315,7 @@ export function ProEmailView({ emailId, client: clientOverride, clientAccountId,
     if (!email) return;
     openComposeTab({
       mode: 'replyAll',
-      replyTo: buildReplyContext(email),
+      replyTo: buildReplyContext(email, writeSource),
       sourceEmailId: email.id,
       sourceEmailSource: writeSource,
       title: buildReplySubject(email.subject || t('email_composer.new_message'), t('email_composer.prefix.reply')),
@@ -316,7 +326,7 @@ export function ProEmailView({ emailId, client: clientOverride, clientAccountId,
     if (!email) return;
     openComposeTab({
       mode: 'forward',
-      replyTo: buildReplyContext(email),
+      replyTo: buildReplyContext(email, writeSource),
       sourceEmailId: email.id,
       sourceEmailSource: writeSource,
       title: buildForwardSubject(email.subject || t('email_composer.new_message'), t('email_composer.prefix.forward')),
@@ -355,6 +365,7 @@ export function ProEmailView({ emailId, client: clientOverride, clientAccountId,
         attachments: [payload.attachment],
         headers: labelSourceHeaders(email.headers),
         originalFrom: email.from,
+        accountId: replyLogin(email, writeSource),
       },
       sourceEmailId: email.id,
       sourceEmailSource: writeSource,