By default, messages in your Inbox are displayed by Date, but you can easily change this to view your messages by conversation, or "thread." In Conversation view, the threads are sorted by date, and then the messages within each thread are sorted based on who replied to whom. You can view the message lists in any of your folders, such as Sent Items or Drafts, this way.
If you arrange your messages in your Inbox by Conversation and you receive a new message, its associated thread moves to the top of your message list.
Note: By default, only unread and flagged messages are displayed. Otherwise, the conversation remains collapsed until you click the arrow next to the conversation heading.
On the View menu, point to Arrange By, and then click Conversation.
A view of an Inbox displaying message threads
1. Click the arrow next to the conversation heading to see all messages in the conversation.
2. This message has no arrow. The message has no replies associated with it.
3. Messages are indented to show you who replied to whom, and when, to help you track the thread.