Here’s how to delete Outlook attachments for Outlook 2016: Fortunately, there are a couple of ways of identifying where the “Outlook Temp” folder that holds attachments in Office 2016 is located. Add in to that the fact that you could be using one of several versions of macOS, it gets messy quickly. The problem with locating them in the Finder is that different versions of Outlook store them in different places. If you use POP 3 it will be all attachments. If you use IMAP, that means those that you’ve opened. However, that only works for attachments that have been downloaded. If you’re thinking that it would be much quicker just to locate attachments in the Finder and delete them all at once, you’d be right.
The slightly quicker, but more complicated manual method
To free up some disk and mailbox space, you can empty the Trash folder. Items in this folder continue to take up space on your computer's hard disk and - if your account is set up to leave copies of the messages on the mail server - in your account mailbox. When you delete messages in Outlook, the messages are moved to the Trash folder.