By default, the folder list in Outlook 2016 for Mac is grouped into similar folders, which means that folders that are common to all your accounts, such as Inbox, Drafts, Sent Items, and Deleted Items, are grouped together.
The Inbox folder at the top of the folder list combines messages received in the inboxes for all your accounts, and then each separate inbox is listed below that. This allows you to quickly and easily check all your email without having to scroll to access your other accounts. You can also access Drafts, Sent Items, and Deleted Items for all accounts combined. The remaining folders unique to each account are listed further down with each account.
NOTE: The On My Computer folder is used for POP accounts. Messages from all POP accounts are downloaded into this folder and stored locally on your computer. However, we recommend you use IMAP for all your email accounts.
However, if you don’t like this view and want to group all folders separately for each account, you can turn off the grouping feature. To do that, open Outlook and go to Outlook > Preferences.
Click “General” under Personal Settings on the Preferences dialog box.
Under Folder list, uncheck the “Group similar folders, such as Inboxes, from different accounts” box. Then, click the “X” in the upper-left corner of the dialog box to close it.
Now, all the folders for each account are listed with the other folders for the same account, instead of grouped with similar folders from other accounts.
One benefit of disabling the grouped folders is that when inbox folders are grouped under the separate accounts, the messages are split into time frames to make it easier to find emails from a specific day or time frame at a glance more easily.
If you use Outlook.com, you can also combine all your email addresses into one Outlook.com inbox.