You can add flags to your emails in Zoho Mail. The flags categories available in Zoho Mail are
You can flag the email directly in the listing pane or from the message pane (preview, popup or tab) in Zoho Mail.
You can flag messages as important in just a single click in Zoho Mail.
Select the emails you want to add flag, by clicking the Checkbox. In the Listing pane command bar, select Mark As and Select the flag you want to apply.
You can remove the flag from the email in the listing pane or in the Reading pane.