Outlook XP: Storing Sent Items on the IMAP Server
By default, Outlook stores a copy of your outgoing email in the “Sent Items” folder in your local “Personal Folders”
Sometimes you’d prefer that it stored them in a folder on the server.
There’s no simple way to do this, so what you have to do is create a rule that processes your outgoing email and makes a copy of it.
See this Microsoft Knowledgebase article for information on achieving this with Outlook 2000. The procedure with Outlook XP is similar.
Unfortunately, it appears that messages placed in “Sent Items” are marked as new, which is a little confusing. I don’t know of a workaround for this.