diff --git a/components/email/__tests__/email-list-scroll-reset.test.tsx b/components/email/__tests__/email-list-scroll-reset.test.tsx
index d6ac07fe5..0ce74a5ed 100644
--- a/components/email/__tests__/email-list-scroll-reset.test.tsx
+++ b/components/email/__tests__/email-list-scroll-reset.test.tsx
@@ -125,7 +125,7 @@ describe('email list scroll position', () => {
       scrollTo.mockClear();
       act(() => {
         useEmailStore.getState().selectMailbox('archive');
-        view.rerender(<EmailList emails={mails} selectedEmailId="m20" />);
+        view.rerender(<EmailList emails={mails} selectedEmailKey="m20" />);
       });
       // The reset comes first and the scroll down to the selected mail after
       // it - the other way round, the reset would leave the mail out of view.
diff --git a/components/email/__tests__/email-list-skeleton.test.tsx b/components/email/__tests__/email-list-skeleton.test.tsx
index 0f6221deb..52cdb77c6 100644
--- a/components/email/__tests__/email-list-skeleton.test.tsx
+++ b/components/email/__tests__/email-list-skeleton.test.tsx
@@ -39,7 +39,7 @@ describe('email list loading skeleton', () => {
     const skeleton = screen.getByTestId('email-list-skeleton');
 
     act(() => { useEmailStore.setState({ mailboxes: [], isLoadingMore: false, hasMoreEmails: true }); });
-    view.rerender(<EmailList emails={[]} isLoading selectedEmailId="x" />);
+    view.rerender(<EmailList emails={[]} isLoading selectedEmailKey="x" />);
 
     expect(screen.getByTestId('email-list-skeleton')).toBe(skeleton);
   });
diff --git a/components/email/__tests__/thread-list-item.test.tsx b/components/email/__tests__/thread-list-item.test.tsx
index 99156b232..712f7d7ff 100644
--- a/components/email/__tests__/thread-list-item.test.tsx
+++ b/components/email/__tests__/thread-list-item.test.tsx
@@ -809,3 +809,22 @@ describe('ThreadListItem hover actions', () => {
     expect(screen.getByTestId('hover-action-delete')).toBeTruthy();
   });
 });
+
+describe('ThreadListItem open message', () => {
+  beforeEach(() => {
+    useSettingsStore.setState({ emailKeywords: [...DEFAULT_KEYWORDS], showPreview: false, mailLayout: 'split', interfaceLayout: 'default' });
+    useEmailStore.setState({ selectedEmailKeys: new Set<string>(), selectedMailbox: '' });
+  });
+
+  // A tag view lists the own and the group's eaaaaab: opening one draws only that one open.
+  it.each([
+    ['own', makeEmail({ id: 'eaaaaab', sourceClientAccountId: 'login-a', sourceAccountId: 'cf' }), true],
+    ['group', makeEmail({ id: 'eaaaaab', sourceClientAccountId: 'login-a', sourceAccountId: 'ce' }), false],
+  ])('draws the %s row as open only when it is the open message', (_name, email, open) => {
+    const [thread] = groupEmailsByThread([email]);
+    const { container } = render(
+      <ThreadListItem thread={thread} isExpanded={false} selectedEmailKey="login-a/cf:eaaaaab" onToggleExpand={() => {}} onEmailSelect={() => {}} />,
+    );
+    expect(container.querySelector('[aria-current="true"]') !== null).toBe(open);
+  });
+});
diff --git a/components/email/__tests__/thread-row-memo.test.ts b/components/email/__tests__/thread-row-memo.test.ts
index 901aaf13b..9217b39a7 100644
--- a/components/email/__tests__/thread-row-memo.test.ts
+++ b/components/email/__tests__/thread-row-memo.test.ts
@@ -64,25 +64,33 @@ describe('areThreadRowPropsEqual', () => {
   it('ignores the open message moving between two OTHER rows', () => {
     const thread = group([email('e1')]);
     expect(areThreadRowPropsEqual(
-      props({ thread, selectedEmailId: 'other-1' }),
-      props({ thread, selectedEmailId: 'other-2' }),
+      props({ thread, selectedEmailKey: 'other-1' }),
+      props({ thread, selectedEmailKey: 'other-2' }),
     )).toBe(true);
-    expect(areThreadRowPropsEqual(props({ thread }), props({ thread, selectedEmailId: 'other-1' }))).toBe(true);
+    expect(areThreadRowPropsEqual(props({ thread }), props({ thread, selectedEmailKey: 'other-1' }))).toBe(true);
   });
 
   it('re-renders the row that gains or loses the open message', () => {
     const thread = group([email('e1'), email('e2')]);
-    expect(areThreadRowPropsEqual(props({ thread, selectedEmailId: 'other' }), props({ thread, selectedEmailId: 'e2' }))).toBe(false);
-    expect(areThreadRowPropsEqual(props({ thread, selectedEmailId: 'e2' }), props({ thread, selectedEmailId: 'other' }))).toBe(false);
-    expect(areThreadRowPropsEqual(props({ thread, selectedEmailId: 'e1' }), props({ thread, selectedEmailId: 'e2' }))).toBe(false);
+    expect(areThreadRowPropsEqual(props({ thread, selectedEmailKey: 'other' }), props({ thread, selectedEmailKey: 'e2' }))).toBe(false);
+    expect(areThreadRowPropsEqual(props({ thread, selectedEmailKey: 'e2' }), props({ thread, selectedEmailKey: 'other' }))).toBe(false);
+    expect(areThreadRowPropsEqual(props({ thread, selectedEmailKey: 'e1' }), props({ thread, selectedEmailKey: 'e2' }))).toBe(false);
+  });
+
+  // Ids repeat across accounts: another account's message with the open
+  // message's id is not drawn as open.
+  it('tells the open message from a namesake of another account', () => {
+    const thread = group([{ ...email('e1'), sourceClientAccountId: 'login-b', sourceAccountId: 'cf' }]);
+    expect(areThreadRowPropsEqual(props({ thread, selectedEmailKey: 'other' }), props({ thread, selectedEmailKey: 'e1' }))).toBe(true);
+    expect(areThreadRowPropsEqual(props({ thread, selectedEmailKey: 'other' }), props({ thread, selectedEmailKey: 'login-b/cf:e1' }))).toBe(false);
   });
 
   it('counts messages shown by the expanded thread as drawn by the row', () => {
     const thread = group([email('e1')]);
     const expandedEmails = [email('e1'), email('e9')];
     expect(areThreadRowPropsEqual(
-      props({ thread, expandedEmails, isExpanded: true, selectedEmailId: 'other' }),
-      props({ thread, expandedEmails, isExpanded: true, selectedEmailId: 'e9' }),
+      props({ thread, expandedEmails, isExpanded: true, selectedEmailKey: 'other' }),
+      props({ thread, expandedEmails, isExpanded: true, selectedEmailKey: 'e9' }),
     )).toBe(false);
   });
 
diff --git a/components/email/email-list.tsx b/components/email/email-list.tsx
index 1d3049a42..011df3db0 100644
--- a/components/email/email-list.tsx
+++ b/components/email/email-list.tsx
@@ -45,7 +45,8 @@ import { MvListSectionHeader } from "./mv-list-section-header";
 
 interface EmailListProps {
   emails: Email[];
-  selectedEmailId?: string;
+  /** emailKeyFor of the open message: ids repeat across accounts. */
+  selectedEmailKey?: string;
   onEmailSelect?: (email: Email) => void;
   /** A mouse was pressed on a row: the click that follows will open it. */
   onEmailPrefetch?: (email: Email) => void;
@@ -125,7 +126,7 @@ function LoadingSkeleton() {
 
 export function EmailList({
   emails,
-  selectedEmailId,
+  selectedEmailKey,
   onEmailSelect,
   onEmailPrefetch,
   onEmailDoubleClick,
@@ -769,10 +770,10 @@ export function EmailList({
 
   // Scroll to the thread group containing the selected email
   useEffect(() => {
-    if (!selectedEmailId) return;
+    if (!selectedEmailKey) return;
     const index = threadGroups.findIndex(thread =>
-      thread.latestEmail.id === selectedEmailId ||
-      thread.emails.some(e => e.id === selectedEmailId)
+      emailKeyFor(thread.latestEmail) === selectedEmailKey ||
+      thread.emails.some(e => emailKeyFor(e) === selectedEmailKey)
     );
     if (index < 0) return;
     // A row clicked in the list is already on screen; scrollToIndex would
@@ -785,7 +786,7 @@ export function EmailList({
     if (item && top !== null && height !== undefined && item.start >= top && item.end <= top + height) return;
     virtualizer.scrollToIndex(itemIndex, { align: 'auto' });
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [selectedEmailId]);
+  }, [selectedEmailKey]);
 
   // Keep the moving end of a Shift+Arrow range on screen (#992).
   const selectionFocusKey = useEmailStore((state) => state.selectionFocus?.focus ?? null);
@@ -1070,7 +1071,7 @@ export function EmailList({
                     {thread && <MemoizedThreadListItem
                       thread={thread}
                       isExpanded={expandedThreadIds.has(thread.threadKey)}
-                      selectedEmailId={selectedEmailId}
+                      selectedEmailKey={selectedEmailKey}
                       isLoading={isLoadingThread === thread.threadKey}
                       expandedEmails={threadEmailsCache.get(thread.threadKey)}
                       onToggleExpand={toggleExpandFor(thread.threadKey)}
diff --git a/components/email/thread-list-item.tsx b/components/email/thread-list-item.tsx
index 4b72cfc76..0634c7334 100644
--- a/components/email/thread-list-item.tsx
+++ b/components/email/thread-list-item.tsx
@@ -299,7 +299,8 @@ function ThreadCountPill({ count, hasUnread, title }: { count: number; hasUnread
 interface ThreadListItemProps {
   thread: ThreadGroup;
   isExpanded: boolean;
-  selectedEmailId?: string;
+  /** emailKeyFor of the open message: ids repeat across accounts. */
+  selectedEmailKey?: string;
   isLoading?: boolean;
   expandedEmails?: Email[];
   onToggleExpand: () => void;
@@ -905,7 +906,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
     loadAttachments,
     thread,
     isExpanded,
-    selectedEmailId,
+    selectedEmailKey,
     isLoading = false,
     expandedEmails,
     onToggleExpand,
@@ -1019,8 +1020,8 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
     const rowTint = accountRowTint
       ?? ((tintListRowsByTag && tagIds[0]) ? tagColor(tagIds[0]).rowTint : null);
 
-    const isSelected = selectedEmailId === latestEmail.id ||
-      thread.emails.some(e => e.id === selectedEmailId);
+    const isSelected = selectedEmailKey === emailKeyFor(latestEmail) ||
+      thread.emails.some(e => emailKeyFor(e) === selectedEmailKey);
 
     const isChecked = thread.emails.some(e => selectedEmailKeys.has(emailKeyFor(e)));
 
@@ -1029,7 +1030,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
         <SingleEmailItem
           ref={ref}
           email={latestEmail}
-          selected={selectedEmailId === latestEmail.id}
+          selected={selectedEmailKey === emailKeyFor(latestEmail)}
           onClick={() => onEmailSelect(latestEmail)}
           onDoubleClick={onEmailDoubleClick ? () => onEmailDoubleClick(latestEmail) : undefined}
           onPrefetch={onPrefetch ? () => onPrefetch(latestEmail) : undefined}
@@ -1571,7 +1572,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
                 <ThreadEmailItem
                   key={email.id}
                   email={email}
-                  selected={email.id === selectedEmailId}
+                  selected={emailKeyFor(email) === selectedEmailKey}
                   isLast={index === emailsToShow.length - 1}
                   onClick={() => onEmailSelect(email)}
                   onDoubleClick={onEmailDoubleClick ? () => onEmailDoubleClick(email) : undefined}
@@ -1613,11 +1614,11 @@ function isSameThread(a: ThreadGroup, b: ThreadGroup): boolean {
 
 /** The open message, if this row draws it; otherwise none of the row's business. */
 function openMessageWithin(props: ThreadListItemProps): string | undefined {
-  const id = props.selectedEmailId;
-  if (!id) return undefined;
-  const drawn = props.thread.emails.some((email) => email.id === id)
-    || (props.expandedEmails?.some((email) => email.id === id) ?? false);
-  return drawn ? id : undefined;
+  const key = props.selectedEmailKey;
+  if (!key) return undefined;
+  const drawn = props.thread.emails.some((email) => emailKeyFor(email) === key)
+    || (props.expandedEmails?.some((email) => emailKeyFor(email) === key) ?? false);
+  return drawn ? key : undefined;
 }
 
 /** Props equality for MemoizedThreadListItem. */
@@ -1626,7 +1627,7 @@ export function areThreadRowPropsEqual(prev: ThreadListItemProps, next: ThreadLi
   for (const key of keys) {
     if (key === 'thread') {
       if (!isSameThread(prev.thread, next.thread)) return false;
-    } else if (key === 'selectedEmailId') {
+    } else if (key === 'selectedEmailKey') {
       if (openMessageWithin(prev) !== openMessageWithin(next)) return false;
     } else if (!Object.is(prev[key], next[key])) {
       return false;
diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index 744aba54f..4771084fd 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -5165,7 +5165,7 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
                 onFindFromSender={mvShell ? (sender) => void handleSelectContactSuggestion(sender, 'from') : undefined}
                 hideBatchToolbar={mvShell}
                 emails={activeEmails}
-                selectedEmailId={selectedEmail?.id}
+                selectedEmailKey={selectedEmail ? emailKeyFor(selectedEmail) : undefined}
                 isLoading={activeIsLoading}
                 hasMore={activeHasMore}
                 isLoadingMoreItems={isScheduledView ? isLoadingScheduled && activeEmails.length > 0 : undefined}