By default Outlook does not validate for empty subject line. This article provides a mechanism to achieve the same.
Introduction
Nowdays, in the professional world, Outlook is used as the email client. Subject line is a very important part of any email. A good subject line can drive the attention of the email reader. How many times has it happened with you that you have sent an email through Outlook without a subject line. After clicking the send button, you realize that you have made a mistake. Are you forgetting to include the subject line? Outlook does not validate for empty subject line.
How to do it?
Follow these steps to validate the empty subject line for Outlook 2007.
1. Go to Tools -> Macro -> Visual Basic Editor. Or directly press Alt + F11.
2. Visual Basic editor window gets opened.
3. On the Left Pane, You can see Microsoft Outlook Objects or Project1, expand this. Now you 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 code pane:
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

Now, just save the project.
One more setting you need to do and that is to enable macro.
Go to Tools->Macro->Security. A dialog window gets opened. Select Warning for all macro option.

That's it.. You are good to go now.
Close your Outlook and open it again. You will be asked regarding the security concern. See the image below.

Click on Enable Macros.
Now try to send an mail with empty subject line. It validates..
Enjoy....