When you use Outlook 2010 and older, you need to use VBA or an add-in to warn about missing attachments.
The following macro needs to go in ThisOutlookSession and runs when the message is sent. If it finds the word “attach” in the message body, it triggers a warning dialog.
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim EndOfMsg, WordAttach As Integer
EndOfMsg = InStr(1, Item.Body, "From:")
WordAttach = InStr(1, Item.Body, "attach", vbTextCompare)
If WordAttach > 0 And WordAttach < EndOfMsg Then
If Item.Attachments.Count = 0 Then
answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
If answer = vbNo Then Cancel = True
End If
ElseIf EndOfMsg = 0 And WordAttach > 0 Then
If Item.Attachments.Count = 0 Then
answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
If answer = vbNo Then Cancel = True
End If
End If
End Sub