I do not get Microsoft's 'design' for this. In Word, Excel, PowerPoint, there is a DIRECT integration with SharePoint Online sites so you can see them very easily. In Outlook, MS decided to allow the SharePoint Sites as an option when inserting attachments, yet only Recent Folders. If you have many folders in many sites, a recent folder list is just plain ridiculous. Even if you could increase teh list to 20+, you would have to open each SharePoint site, then each folder in each doc library, in order to grab a file from it in Outlook to send an email.
THIS MAKES NO SENSE. If I can open up word, create a document, then File > Save As or File > Open and quickly browse to any Site, I should be able to do this in Outlook. Microsoft does realize that people want to keep their SharePoint files for internal users only, NOT require 500Gb+ worth of data to be synchronized locally, in order to email a file to someone?