By default, when you add most account types to Outlook 2013, 2016, or Outlook for Office 365, your messages are displayed in Conversations. A conversation includes all messages in the same thread with the same subject line. Older versions of Outlook may not show conversation view enabled by default.
From any email folder, such as your inbox, click View > Show as Conversations to toggle Conversation view on or off.
Click All mailboxes or This folder.
Conversation that include multiple messages are identified in the message list by . Click to expand the Conversation.
Within each Conversation, messages are sorted with the newest message on top. You cannot change this sort order.
When you receive a new message that’s part of a Conversation, the whole Conversation moves to the top of your message list.
Any Conversation that includes unread messages has a bold subject and the count of the unread messages appears next to the subject. When you click to expand the Conversation, any unread message has a bold heading.
Change Conversation options
There are several conversation options you can change.
From any email folder, such as your inbox, click View > Conversation Settings
Select any of the available options.
Show Messages from Other Folders Use this option to display messages in the conversation that have been moved to other folders as well as messages you've sent that are stored in your Sent Items folder.
Show Senders Above the Subject Selecting this option shows the sender names at the top of the conversation rather than the conversation subject.
Always Expand Selected Conversation This option applies only to the currently selected conversation. The conversation selected when you check this box will always appear expanded.
Use Classic Indented View Classic indented view shows messages in the conversation indented based on their position within the conversation.
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.