At this point you can drag and drop entire folders of email from Outlook onto the IMAP server name. This will copy the email, including all attachments, to the Linux box. Unfortunately it also immediately deletes it from Outlook. For the destination, pick your Linux server at the bottom of the list. However, life isn't quite that simple.
Outlook supports folders containing folders which also contain messages.
So if you have folders in Outlook with both messages and subfolders, you can't copy the entire tree over to the Linux IMAP server. Another incompatibility of the Linux IMAP server is that you have to tell it in advance if a new folder will contain subfolders or messages. This slash will disappear when the folder is created. So, in order to copy a tree of folders to the Linux IMAP server, first you need to create a replica of the structure of your existing folders on the Linux IMAP server. While you're doing this, note which of the existing folders contain both subfolders and messages.