Outlook storage issues

I had the same experience and took me a month to find a solution.

The New Outlook downloads all mail from the mail service again, and also duplicates all the "On My Computer" local emails from the Legacy Outlook. All the emails in the Legacy are still there. That is why the "/Library/Group Containers/UBF8T346G9.Office/Outlook" folder takes up twice the space.

Switching back to the Legacy Outlook does not delete the emails in the New Outlook.

Even within the same Outlook, deleting emails does not necessarily reduce the disk space it occupies.

My final solution is as follows:

1. Back up the whole mail account using "File > Export" and choose all categories in "Items of these types"; Email, Calendar, Contacts, etc. If your local disk is too full, use an external disk
2. Uninstall Outlook by moving Outlook from the Application Folder, then empty Trash
3. Delete the folder "..../Library/Group Containers/UBF8T346G9.Office/Outlook" then empty Trash (This is where the bulk of the storage is)
4. Reinstall Outlook (It will start in the "New Outlook" mode)
5. Log in to your mail account (it will start downloading emails from the mail server), then switch back to "Legacy Outlook" using "Outlook > Legacy Outlook" quickly. This will avoid the New Outlook backing up all the local emails under "On My Computer" of the Legacy Outlook (Note: how to switch back to Legacy model may vary depends on the version)
6. Now you are in the Legacy Outlook, import your previous emails using "File > Import" and choose "Outlook for Mac archive file (.olm), then import the file created in Step 1 above
7. Now the storage taken by the "New Outlook" should be limited, and all emails are restored in the "Legacy Outlook". Disk usage should drop back down to a bit more than half of what it was
8. Note that all emails in the "live" mailbox (sync'ed with the mail server) are restored under "On My Computer" during import, while these emails are also being sync'ed from the server to Outlook. After confirming there is a duplication, you may want to deleted those duplicated emails from "On My Computer" to save further disk space

I took me a quite a while to resort to this, and I hope this will help solve your problem.

The Legacy Outlook will still sometimes on relaunch automatically switch to the New Outlook. It is possible in the New Outlook to switch back to the Legacy Outlook but the issue of doubling mailbox size comes back if not done swiftly. There does seem to be a way to disable the New Outlook. So that the Legacy Outlook application no longer shows the toggle option to switch to the New Outlook in main window. It also seems to remove the option in the Menu bar items list and prevent the automatic switching to New Outlook when relaunched.

1. Make sure you have switched to the Legacy version of Outlook.

2. Quit Outlook and open Terminal

3. Type into Terminal the command: defaults write com.microsoft.Outlook EnableNewOutlook -int 0

4. Hit enter in Terminal and open Outlook. Notice the Enable New Outlook toggle is gone from the user interface.

If in Terminal you instead added the command "defaults read com.microsoft.Outlook" before doing the above steps you would see the entirety of the current settings in "com.microsoft.Outlook". By default there does not seem to be a line with a setting for "EnableNewOutlook". It would seem that by default the Outlook app if it does not have a setting specified the app assumes the setting is true.

Afer you followed the above 4 steps you can confirm the setting by the command "defaults read com.microsoft.Outlook | grep EnableNewOutlook". If the setting exists in the file then you will see the single line returned from the file using the grep command.

If you want to undo this setting change and be able to see the New Outlook toggle again then in Terminal you can type the following command which sets the setting to true.

defaults write com.microsoft.Outlook EnableNewOutlook -int 1

Hopefully this helps avoid unintended automatic switching to the New Outlook.

It is generally a good computer practice to alway keep at least 15% to 20% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur  

The links below will assist in identifying what is taking up space on the Internal Drive and provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac.  

Often caused if the Time Machine Drive has not been attached  to the computer and TM Backup is set to run on a Schedule. 

TM Backup will make Snap Shot on the Internal Drive awaiting the TM Backup Drive to be attached. 

Only then will the Snaps Shots be transferred to the External Drive and deleted the Internal Drive.

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - and https://bombich.com/kb/ccc6/automated-maintenance-ccc-safetynet-folder suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

sridharan273 wrote:

Please also advise how to prevent system data growing to such sizes as to consume the entire storage.

I recommend you contact Microsoft support or ask a question in the Microsoft support forums. You've found the one place on the internet where people are the least likely to use Outlook.

sridharan273 wrote:

Sorry I took a significant time in responding.

I believe the issue is with Apple and not Microsoft. I was able to check the folders within the Library folder, and found that more than 75% of the space in the System Data was taken up by the Outlook files.

Sorry but the assertion that the issue belongs to Apple is somewhat misguided by virtue to the later part of your above statement .

As had been suggest by another Contributor, Microsoft software on Apple Operating System is not an Apple issue but rather a Microsoft Issue.

Apple Supports Apple Software, devices and Service as it owns these products or services.

Likewise, Microsoft owns and operates the software in question and should support questions related to Their Products

sridharan273 wrote:

Sorry I took a significant time in responding.

User wrote " I believe the issue is with Apple and not Microsoft. "

Apple can fix Apple Services, Applications, Operating Systems and Accessories 

Apple Can not fix Third Party Services that use Third Party Applications 

Apple can not fix the choices a user chooses to install when it comes to choices of Third Party Applications.

That falls clearly Out-Side the Domain of Apple and rests with Third Party Application Developers