Cannot set extended property on a folder under the message-folder root

[MAnantharaman]

In debugging an issue where exchange-folders weren't being presented to an IMAP-client I used EwsEditor to recognize that the folders affected didn't have the FolderClass property (which should be there and set to IPF.Note). I tried setting that property for a quick test but always get an exception. See attached dialog to set the property and the exception. I ended up changing FolderClass with Powershell over EWS Managed API - which I would have had to do anyway because many folders were affected - but being able to "prototype" my fix in EwsEditor would have been helpful ...