|Outlook Developer Reference|
expression A variable that represents a MailItem object.
This property corresponds to the MAPI property PR_ENTRYID.
A MAPI store provider assigns a unique ID string when an item is created in its store. Therefore, the EntryID property is not set for an Outlook item until it is saved or sent. The Entry ID changes when an item is moved into another store, for example, from your Inbox to a Microsoft Exchange Server public folder, or from one Personal Folders (.pst) file to another .pst file. Solutions should not depend on the EntryID property to be unique unless items will not be moved. The EntryID property returns a MAPI long-term Entry ID. For more information about long- and short-term EntryIDs, search http://msdn.microsoft.com for PR_ENTRYID.