Try this:
To create the macro in Outlook 2003, go to Tools -> Macro -> Visual Basic Editor (this feature has to be installed). In Microsoft Office Outlook Objects -> ThisOutlookSession, paste the following code:
Private Sub Application_ItemSend _
(ByVal Item As Object, Cancel As Boolean)
Dim strMessage As String
Dim lngRes As Long
If Item.Subject = "" Then
Cancel = True
strMessage = "Please fill in the subject before sending."
MsgBox strMessage, _
vbExclamation + vbSystemModal, "Missing Subject"
Item.Display
End If
End Sub