MAPI Error: "Error Occurred Retrieving SOURCE_KEY Property"

Riva reported a MAPI_E_NOT_FOUND access permission error for a user mailbox that was previously syncing:

Possible causes

• The Riva connection user for the EWS connection has been added to the AD Domain Admins group.  If this happens, the same error would happen for all of the syncing users.

• Delegate Full Access permissions has been removed for the Riva connection user, so that impersonation will fail.  Check assigned permissions:

  • Configure Delegate Full Access permissions in Exchange management Console
    
    OR
  • Configure Delegate Full Access permissions in Exchange Management Shell.

Cause

Examination of the user mailbox in Exchange confirmed that the Delegate Full Access permissions to the Riva connection user for EWS had been removed.

Resolution

Once the permissions were re-assigned, data sync resumed as expected.