One of our clients has been receiving bouncebacks when sending to certain clients. Bounceback details shown below,
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
<***********.local #4.0.0 smtp;451-You have been greylisted. This is part of our standard anti-spam measures>
Our client is using exchange 2003 on 2003 server.