Outlook groups conversations using several properties beyond Message-ID, References, and Reply-to. It also considers In-Reply-To headers and a hidden property called Conversation Index to track email relationships within a thread. If headers are missing or modified, Outlook may fall back on subject similarity and sender information to group emails. Make sure to check the Conversation Index and In-Reply-To fields for consistency. These additional factors should improve the accuracy of email grouping in conversation view.