Favorites

This section starts with the Inbox, Sent Items, and Deleted Items of your main mailbox. You may also see Office 365 Groups that you are a member of. You can add to the Favorites section by dragging and dropping from anywhere, such as the Inbox of other mailboxes or Office 365 Groups, if any, listed in the List of email accounts (#10) section:

You can also remove a portion of a mailbox by right-clicking on it and choosing Remove from Favorites:

The line between this section and the list of emails section can be dragged to give more or less space to the viewable portion of the section.