Message details: "for pdc.domain.com (<IP address>)."- These errors result when spammers or spoofers attempt to use an open relay on an Exchange server to send mail. The error occurs when Exchange cannot authenticate the host or client trying to send the mail. You should receive these messages if you have "hosts and clients that successfully authenticate" checked in IMS routing restrictions. The errors indicate that your relay is secure.
Message details: "<email address> for xxx.xxxx.net.xx(<ip number>)" - From a newsgroup post: "If the exchange server is behind a PIX firewall check the PIX smtp handling - If it's enabled, it kills the auth login every time, so external clients can't send through the server even if they
are set to authenticate."