I am currently running SBS 2003 server and have an issue with a few unauthorized third party servers sending email using our SMTP Virtual Server. Its currently set to allow anonymous with authentication and integrated windows authentication. I need to keep these servers from being able to send email out from our SMTP but also allow end users to be able to send/receive. When I remove the anonymous option and leave only integrated windows authentication, the server isn’t able to receive email. I confirmed in Microsoft’s knowledgebase this is by design. Does anyone else have some good steps for locking out these bad servers and not interfering with your network users abilities to send and receive? I thought about blocking the IP, but they are dynamic it appears.