This function will prompt for confirmation if you try to send an email with no Subject.
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim strSubject As String
strSubject = Item.Subject
If Len(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
Add it to the “ThisOutlookSession” part of the VB Code. See the source below for full details
Source: http://sarathc.wordpress.com/2006/06/29/forgot-to-mention-subject-in-outlook/