Outlook using most of the storage in Macbook Air

There is no easy answer. Outlook 2016 for Mac stores stores messages, calendar events, contacts, tasks and other data in a SQLite database in the /Users/username/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles folder.

You can't just copy or move this folder the way you used to be able to copy or move the Microsoft User Data folder in Outlook 2011.

When I looked into the possibility of using a symbolic link to relocate the Outlook 2016 folder to a different drive all the info I could find warned me off from doing it. One of the issues is that Catalina is not especially friendly to symbolic links that involve the Desktop & Documents folders (it reportedly deletes the symlinks when you reboot). So I can't confirm whether or not it would work. You could try it but make sure you back up everything first, thoroughly explore how to copy the folder & create a symlink and make sure no part of Outlook or Office is running when you do it. Make sure you have that backup because you may very well need it.

There's some information in this thread on the MS website: Where Does Outlook 2016 store its data?

You might consider exporting less-used folders to MBOX files for backup and to reduce the size of your Outlook data file. 70GB is awfully big.

To add to MartinR's excellent post:

I tried using a link for Office 2016 when I was using that, just for the sake of testing. Outlook would launch, but then sort of freeze. I tested it again just now with the current version of Office 365. If I move the entire UBF8T346G9.Office folder to another location and replace it with a link, Outlook won't even launch. If I move only the subfolder Outlook within UBF8T346G9.Office and replace the it with a link, I get this message:

I'd say it can't be done.

You can drag and drop emails out to the desktop or other location and then remove those email from Outlook. Double clicking any separated copy opens in Outlook. However! You will lose any attachments doing this. They are not saved with the .eml file the drag/drop copy creates.

The only way I can see to get the size down is to:

1. Drag emails out to the desktop you want to save, but don't need to be in the main database anymore. Each .eml file's name will be its topic title in Outlook.
2. Drag out any attachments for an email that has them.
3. You'll have to create an Excel spreadsheet or something to keep track of which attachments belong with which .eml files.
4. Delete the email in Outlook you made an external copy of.

You can then keep adding to this external folder of items as needed. Make sure to have more than one backup of these copies on separate drives.