A friend of mine forward me this useful tips for MS Outlook user. This works in MS Outlook 2003, but the steps may different on Outlook 2007

Steps: -

1. Open your Microsoft outlook
2. Press Alt+F11. This opens the Visual Basic editor and then Press Ctrl+R which in turn opens Project-Project 1 (left side).
3. On the Left Pane, one can see “Microsoft Outlook Objects” or “Project1″, expand this. Now one can see the “ThisOutLookSession”.
4. Double click on “ThisOutLookSession”. It will open up a code pane.
5. Copy and Paste the following code in the right pane. (Code Pane)  and save it.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim strSubject As String
strSubject = Item.Subject
If Len(Trim(strSubject)) = 0 Then
Prompt$ = “Subject is Empty. Are you sure you want to send the Mail?”
If MsgBox(Prompt$, vbYesNo + vbQuestion + vbMsgBoxSetForeground, “Check for Subject”) = vbNo Then
Cancel = True
End If
End If
End Sub

Keep in mind, to be able to run this script(message box) everyday, please enable your outlook macros.

To do so, go to Tools > Macro > Security.
Set the Security Level to medium.