Alert for Empty Subject Line in Outlook Express

 Just follow below steps,
1. Open the Outlook
2. Open Visual basic editor by pressing ALT+F11
3. Then from left pane expand the Project1 untill you see "ThisOutlookSession"
4. Copy & Paste the below code in "ThisOutlookSession" Window.
6. Save and Close the window. It's done!

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)

    If Item.Subject = "" Then

        'Edit the message and the popup caption on the next line as desired.'

        MsgBox "You are not allowed to send an item with a blank subject.  Please enter a subject and send again.", vbCritical + vbOKOnly, "Prevent Blank Subjects"

        Cancel = True

    End If

End Sub