When you attach a file from your OneDrive account to an email message, by default, the recipient will be given a link to the online file, which may cause some difficulties if the recipient hasn’t been given permission to the file or if the recipient is outside of the university. To bypass potential problems, you can attach a copy of the file to the message.
To attach files from your OneDrive account when composing emails in Outlook for Mac, you first need to sync your OneDrive data to your Mac using the OneDrive app.
Once you’ve done that:
Add a OneDrive attachment to a message in Outlook Web Access (OWA)
Add a OneDrive attachment to a message in Outlook for Windows