Outlook macro to interrupt a reply-all?

Note: I'm working on 2007, but I think the code should transfer OK.

You can add an event handler via VBA to pick up the ReplyAll event. Something like the following:

Dim WithEvents insp As Outlook.Inspectors
Dim WithEvents mailItem As Outlook.MailItem

' This is called on Outlook startup
Private Sub Application_Startup()
    Set insp = Application.Inspectors
End Sub

' This is called when a new Inspector is created. You use it to pick up on a new mail item event
Private Sub insp_NewInspector(ByVal Inspector As Inspector)
    If Inspector.CurrentItem.Size = 0 And Inspector.CurrentItem.Class = olMail Then
       Set mailItem = Inspector.CurrentItem
    End If
End Sub

' Called when you press ReplyAll
Private Sub mailItem_ReplyAll(ByVal Response As Object, Cancel As Boolean)
    Dim msg As String
    Dim result As Integer
    msg = "Do you really want to reply to all?"
    result = MsgBox(msg, vbYesNo, "Reply All Check")
    If result = vbNo Then
        Cancel = True
    End If
End Sub