If you want to do this without code, select Options from the View menu, and
click the Browse button in the "Save sent message to" section and choose a
different folder to save a copy of the sent message. This is most useful of
course when you folder that you want to save a copy to constantly changes.
However, you can use code to change the default folder a message is saved to
(there's a global setting to save these messages to the Sent Items folder).
Here's a great example from the Outlook VBA Help file:
--
This Visual Basic for Applications (VBA) example sends a reply to Dan Wilson
and sets the SaveMyPersonalItems folder as the folder in which a copy of the
item will be saved after being sent. To run this example without errors,
make sure a mail item is open in the active inspector window and replace
'Dan Wilson' with a valid recipient name.
Sub SetSentFolder()
Dim myItem As Outlook.MailITem
Dim myResponse As Outlook.MailITem
Dim mpfInbox As Outlook.MAPIFolder
Dim mpf As Outlook.MAPIFolder
Set mpfInbox = Application.Session.GetDefaultFolder(olFolderInbox)
Set mpf = mpfInbox.Folders.Add("SaveMyPersonalItems")
Set myItem = Application.ActiveInspector.CurrentItem
Set myResponse = myItem.Reply
myResponse.Display
myResponse.To = "Dan Wilson"
Set myResponse.SaveSentMessageFolder = mpf
myResponse.Send
End Sub