With NetDocuments it is possible to map folders within Outlook to NetDocuments. This requires NDMAIL to be purchased and installed as part of your ND subscription.

Select the Workspace you would llike to sync emails to 

Then make the necessary changes to the pop up screen

How to force a RE-SYNC of ND Mapped folders. 

click on the button 're-sync' and see if you still get the error.

Failing that please repeat the process but this time remove the folder mapping by clicking gon 'UNLINK'  and then re-mapping the same folder