Microsoft Outlook files present a problem to sync programs because the files are always in use and constantly being updated as the users send/receive emails, change email statuses from unread to read, or many other events. If you simply "attach" the Outlook directory (usually: %localappdata%\Microsoft\Outlook), then the Windows Client will generate many sync failures because it will be denied access to the files unless the Microsoft Outlook program is closed.
One way to get these files to sync to the cloud is to enable "scheduled sync" at either the Tenant or Cluster level. To do this, go to Group Policy->Folder & Storage->Attached Folder and enter the following text:
Next, open the Group Policy->Folder & Storage->Filters page and change the default filter to something else. This is necessary, because if you simply remove all extensions, the filter will default to the exclusion of PST files. So, in order to not filter out any extensions, enter a non-existing extension, such as [.ext1]
For an explanation about the Microsoft Outlook files, please go to https://support.office.com/en-us/article/introduction-to-outlook-data-files-pst-and-ost-222eaf92-a995-45d9-bde2-f331f60e2790
After you've applied the settings, please remove any sync failures related to PST and OST files and restart the Windows Client for the setting to take effect. You can manage the Windows Client tasks from either the Native or Web Management Consoles.
To open the Native Management Console, click on the cog wheel icon of the Windows Client menu while pressing the control key on the keyboard.
Delete any PST or OST tasks that are pending or failed:
To do the same thing via the alternate Web Management Console, click on the Management Console option of the Windows Client options menu. Then, click on the device menu (step 3 on the screenshot below) and select Task Manager.
Remove the pending or failed PST or OST tasks:
After you've removed all the pending or failed tasks from either the Native or Web Management console, please restart the Windows Client. After it restarts, a System Delayed Task will be created and will attempt to sync the files at regular and infrequent intervals (the default value is 5 minutes) as opposed to every time the Outlook files change. The delayed sync tasks will use Shadow Copy in order to access the files even if they are being held hostage by the Outlook program, so you should not get sync failures. Eventually the files will be uploaded to the cloud without errors.
If you still have sync issues after this, please open a support ticket by sending an email to firstname.lastname@example.org.