When ItemSend event fires, the message has not been submitted yet (you can set the Cancel parameter to true). The earliest you can see the message in the sent state is when the Items.ItemAdd event fires on the Sent Items folder.
Why not simply set the MailItem.SaveSentMessageFolder property? Outlook will automatically move the message after it is sent. The only limitation is that the folder must be in the same store where the message was created.