By default, Outlook for Windows and Outlook for Mac displays your messages in order of Date Received. You can change the sort order by clicking the heading in any column, or by selecting an option under the View menu, Arrange By.

By default, Outlook on the Web displays your messages with Conversation View turned on

Arranging your messages in Conversation View clusters your messages based on Subject. The original message, your replies, and other people's replies are all displayed together.

Conversation view can help by giving you a compact record of an entire thread, but it can be confusing if you have more than one conversation with identical Subjects, or if you prefer to see your messages in strict chronological order.

Conversation View in Outlook for Windows or Mac

View Email Messages by Conversation in Outlook for Mac

View Email Messages by Conversation in Outlook for Windows

Conversation View in Outlook on the Web

To turn Conversation View on or off,

1. In Outlook on the Web, on the red, Cornell-themed toolbar, click the gear icon, then click View all Outlook settings.
2. In the left column, click Mail.
3. In the center column, click Layout.
4. In the right column, under Message organization, choose the setting you prefer. There is no "save;" the change takes effect immediately.
5. Close the Settings panel to return to your mail.