Microsoft Outlook automatically groups items by date. However, you can also group items manually by using standard arrangements or by creating your own custom grouping. While you may be most familiar with the default groups in your Inbox, these groups are also applied to other mail folders.
The default groups that are displayed in your Inbox include Today, Yesterday, Last Week, Last Month, and Older. Some of the standard arrangements that are available include grouping according to Categories, Size, Subject, and Importance.
If you want to group items by Conversation, select the View menu, then check the box for Show as Conversations. For more information about conversations, see view email messages by conversation.
Group or ungroup items automatically
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To add or remove grouping in an arrangement, on the View menu, in the Arrangement section, select Arrange By.
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In the menu, select Show in Groups.
Group items manually or create a custom group
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On the View menu, in the Current View group, select Change View, and then select Save Current View As a New View.
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Type a name for the new view, and then select OK.
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On the View menu, in the Current View group, select View Settings.
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Select Group By, then clear the Automatically group according to arrangement check box.
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In the Group items by box, select a field to group by.
If the field that you want is not in the Group items by box, select a different field set in the Select available fields from box.
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Select Ascending or Descending for the sort order of the group headings.
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To display the field that you are grouping items by, select the Show field in view check box.
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To group by subgroups, select a field in the Then by box.
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In the Expand/collapse defaults list, select the default for how you want groups to display in the view.
After closing the dialog box, display or hide items in a group by selecting Expand 
Ungroup items manually
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On the View menu, in the Current View group, click View Settings.
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Click Group By.
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In the Group By box, clear the Automatically group according to arrangement check box.
