Write code for the Application_ItemSend event handler, e.g.:
Private Sub Application_ItemSend _
(ByVal Item As Object, Cancel As Boolean)
If Item.Subject = "" Then
Cancel = True
MsgBox "Please fill in the subject before sending.", _
vbExclamation, "Missing Subject"
End If
End Sub
For a more elaborate version that also checks for expected attachments, see
http://www.outlookcode.com/codedetail.aspx?id=553
And see
http://www.outlookcode.com/d/vbabasics.htm if you're just getting started with Outlook VBA.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers