Is there any way to have a secure SMTP mail service that will prevent relay of mail by unauthorized users. We have an Exchange server that has POP3 and SMTP mail services setup for external users who access the mail server from our hosted ISP. We have indication that the service is being used as a mail relay for other busineses. We have tried many configurations most of which either allow relay or totally deny the service. Any help on this subject would be greatly appreciated.
