Encontrado, e inclui uma verificação para impedir o envio para uma lista específica:
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim str As String
Dim j As Integer
Dim splits() As String
Dim lbadFound As Boolean
lbadFound = False
Set Recipients = Item.Recipients
receivers = Recipients.count
For i = receivers To 1 Step -1
Set recip = Recipients.Item(i)
If InStr(1, LCase(recip), "lists.bla.com") >= 1 Then
lbadFound = True
End If
Next i
splits = Split(Item.To, ";")
If receivers > 1 Or lbadFound Or UBound(splits) > 0 Then
If MsgBox("Do you want to continue sending the mail?", vbOKCancel) <> vbOK Then
Cancel = True
End If
End If
End Sub