Messages are displayed in message lists, such as your Inbox by Date, but you can easily change this to view your messages by thread or "conversation." The threads are sorted by date, and then the messages within each thread are sorted based on who replied to whom.
Find all messages in the same thread
On the View menu, point to Arrange By, and then click Conversation.
Tip: If you are viewing your Inbox in multi-line layout, you can quickly change from one arrangement to another by right-clicking the Arranged By column heading, and then clicking the arrangement you want.
If you arrange your messages in your Inbox by Conversation, and you receive a new message, the entire thread that it is a part of 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.
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 they replied to help you track the thread.