1. Open the appropriate Outlook folder.
2. Right click any of the field names.
3. Click Arrange By and then select conversations now select Show Messages in Conversations so that the Show Message in Conversations is no longer checked.
To apply this change to all Outlook folders:
1. Select the View tab on the Ribbon.
2. Click Change View.
3. Click Apply Current View to other mail Folders.
4. Select your entire mailbox and click OK.