The recover mailbox data wizard does not support filtering of data. Therefore, you always merge or copy all items in the RSG back to the production mailbox. With ExMerge you can use the Data Selection Criteria dialog to restrict the items that are recovered to the production mailbox.
Let’s assume the following scenario:
A user is cleaning up his inbox and deletes several hundred mails. The deleted item retention period expires. The user recognizes that he needs one of his deleted mails back. You have to restore a previous backup to the RSG. With ExMerge you can recover just this important mail. With the recover mailbox data wizard you would not want to merge the content of the inbox from a few weeks ago with the current one, because ALL deleted items would reappear. If you use the copy function then you would “double” the mailbox size by importing all old content into the created “Recovered Data date / time” folder.