Using older versions of Outlook for Mac in business environments
I didn't write down the detailed steps to doing this, but used this post to lead me down the right path to importing the messages in bulk. If memory serves...
- I dragged/dropped Outlook sent items into individual sub-folders that I organized by time period. (FYI: There's a limit on how many items Outlook allows you to drag/drop. Given the fact I had several years of sent items that I was doing this for, I needed a Finder folder structure that would me stay organized and not miss anything.)
- Once all the messages were in their individual sub-folders, I added the ".mbox" extension to the sub-folder name. Example, the folder "iCloud Sent Items for Apr 2020-Sep 2020" became "iCloud Sent Items for Apr 2020-Sep 2020.mbox".
- In mail.app, I used the Import -> Files in mbox format" feature to import emails by Finder sub-folder.
- Then I *think* those messages got imported into an "Import" folder in mail.app, which I then moved to "Sent" folder.
I hope this helps you!