Use this method if it doesn't matter if someone other than the recipient might get to view the contents of the attachment. For this reason, if you need to send a file as an attachment to an external user consider these alternatives:
not part of your school), then requiring them to sign in to access a file may not be the smoothest process when attaching and sharing a file using Outlook. You will see that if your recipient is an external user (i.e. If you want attach a file from OneDrive, please see Attaching a File in Outlook from OneDrive, but you'll notice that it is a very similar process. This article will show you how to attach a file from a modern SharePoint team site to an e-mail that you are sending from the Outlook web app. For an overview of attaching files from SharePoint, and an explanation of the different choices you can make depending upon exactly what you are sharing and to whom, please see SharePoint - Attaching Files to E-mails and External Sharing
