block user to send mail to public domain

From http://www.slipstick.com/how-to-outl...email-address/

"This macro checks messages for one specific address and if it finds the address, allows you to cancel the send. On Error Resume Next allows it to work with meetings or task requests, otherwise it kicks up an error message."

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
     
On Error Resume Next
 ' use lower case for the address
 ' LCase converts all addresses in the To field to lower case
If InStr(LCase(Item.To), "[email protected]") Then
      Prompt$ = "You sending this to " & Item.To & ". Are you sure you want to send it?"
       If MsgBox(Prompt$, vbYesNo + vbQuestion + vbMsgBoxSetForeground, "Check Address") = vbNo Then
         Cancel = True
       End If
  End If
     
End Sub

To make this work for you "[email protected]" becomes "@publicdomain.com"

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
     
On Error Resume Next
 ' use lower case for the address
 ' LCase converts all addresses in the To field to lower case

If InStr(LCase(Item.To), "@publicdomain.com") Then
    Prompt$ = "Sending to @publicdomain.com will be cancelled."
    MsgBox Prompt$
    Cancel = True
End If

End Sub

This macro needs to be added to ThisOutlookSession to work.