You can mark the conversation unread, of course, or star it perhaps — or rely on a hidden Gmail gem that lets you mark a thread unread only from a certain message onward.
Mark Part of a Conversation Unread in Gmail
To mark as unread part of a thread — or just the latest message — in Gmail:
- Open the conversation in Gmail.
- Make sure the message you want to be the first to be marked unread is expanded.
- If you cannot see the message, click its sender's name and preview.
- You can also select Expand all to the right of the thread.
- Click the down arrow next to Reply in the message's header area.
- Select Mark unread from here from the menu.
You can also mark the entire thread unread, of course, by pressing Shift-U.
Mark Individual Emails Unread in Gmail
To mark an individual email messages unread in Gmail:
- Make sure conversation view is disabled.
- Find and check or open the desired email.
- Press Shift-U.
- You can also select More actions | Mark as unread from the toolbar.
(Updated November 2011)