I have read of all the ways SIS works but one, and was wondering if anyone has an answer. I know it works on both messages and attachments (at least Exchange 2003 and before). The question I have is for the attachments. If a user were to send an attachment to another user, then that seconduser saved the file to disk, then at a later time that second user sent the file as an attachment to a third user, would SIS store only one copy of the attachment or two. This is under the assumption of one message store. Thanks, Hans Schroeder

My understanding is that as long as it's the same attachment, it's stored only once in the store. If the attachment is altered by the second user and then sent to the third user, nowyou have 2 different attachments stored in the store. CB

When the file is attached to a new email (from disk) and sent, even if the original file is unmodified from the previous sender, it would still be stored in Exchange as a new object. The SIS simply uses pointers to identical objects in the database based on Exchange's knowledge of the object. Exchange would have no way of knowing that the new attachment is essentially the same file as one already stored in it's database from an earlier message; to do so would require a full DB search each time an object was attached and delivered, which would be terrible for message transfer performance.

Hi Hans, I agree with Jeff's comments. Here is a article about Single Instance Storage in Exchange 2007 for your reference: http://msexchangeteam.com/archive/2008/02/08/448095.aspx Regards, Elvis