Deleting a Property
Outlook provides several ways to remove a custom property:
Removes a custom property specified by Index in the UserProperties collection for the item. The UserProperties collection is one-based.
Removes a custom property specified by Index in the ItemProperties collection for the item. The ItemProperties collection is zero-based. You can only remove custom properties in the collection and they are denoted by IsUserProperty. You cannot remove explicit built-in properties.
Removes a custom property specified by SchemaName, provided that the property is not read-only and the caller has permission to delete the property (for example, the caller is the owner of the folder to which the property has been added). You cannot remove a built-in Outlook or MAPI property.
For each custom property in SchemaNames, removes it provided that the same conditions described in the PropertyAccessor.DeleteProperty column are true. Any error will be returned in the corresponding element in the resultant error array.
All Outlook item objects except Microsoft Office document items (DocumentItem objects)
All Outlook item objects but excluding the DocumentItem object, and any of the following objects: AddressEntry, AddressList, Attachment, ExchangeDistributionList, ExchangeUser, Folder, Recipient, and Store objects.
Same objects as listed in the DeleteProperty column.