Exchange 2007 introduced a new URL format (Constructing OWA 2007 item ids from WebDAV items) which contained an arbitrary item id, which was based on the EntryId of the item. The format was this:
| Length | Meaning |
| 1 | Length of the structure |
| sizeof(EntryId) | EntryId |
| 1 | Item type |
This format changed with Exchange Service Pack 1. The layout is now this:
| Length | Meaning |
| 4 | Length of the user's email address |
| sizeof(EmailAddress) | Email address specifying the mailbox which contains the item |
| 4 | Size of the EntryId |
| sizeof(EntryId) | The EntryId of the item |
This layout also applies to folder ids within a mailbox.