Not all the email are being rejected. We have just dicovered that 2003 Exchange requires a Auth XEXCH50 reply from the sender. If this reply is not Auth by the exchange server the email is drop. See MS 818222
We have verified the A and MX records. They are a pointed the the correct sever and IP. That's what has been puzzling use. If the record is correct and pointed, is Exchange stopping the receiving We are also ruuning Symantec Mail Client, we stop the services on that. No difference.