Mail Manager
Importing emails filed with Mail Manager is both fast and simple via the Bulk Uploader. Be sure to take a look at the Quick Start as the example import is with Mail Manager content.
By default, the Bulk Uploader sets the Import source: to Collection files but be sure to check that you have the correct source selected.
Set the Root Folder: to where your collection file/s are.
Collection files are shown in the left-hand list. When you select a collection, the details from that file are shown on the right.