There is no such feature, but you can build it in with a little VBA code:
Private Sub Application_ItemSend _
(ByVal Item As Object, Cancel As Boolean)
Dim strMsg As String
Dim res As Long
If Item.Subject = "" Then
Cancel = True
strMsg = "Please fill in the subject before sending."
MsgBox strMsg, _
vbExclamation + vbSystemModal, "Missing Subject"
Item.Display
End If
End Sub
For a more elaborate version that also checks for expected attachments, see
http://www.outlookcode.com/codedetail.aspx?id=553
For VBA basics, see
http://www.outlookcode.com/d/vbabasics.htm
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers