All items in Outlook are uniquely identified by a combination of the
EntryID of the item. This is a constant unless the item has been moved
(it remains the same if moved in a PST file) in an Exchange mailbox or
public folders store. In addition, each item has a ConversationIndex
property that in combination with the ConversationTopic property.
Those could be used.
Other than that you could generate your own unique series of tags for
items by using a database table's primary key property or by storing
an item in a folder, say a Post item, that has a seed number that is
incremented each time the seed number is used.
