I am running Exchange on an installation of SBS 2003 and I have recently run into a problem where random spam queues began showing up in my SMTP queues. I was first alerted to it because my antivirus software was telling me my server was infected, but luckily I think it's just because the spam contains malicious attachments. One weird thing is that all of the messages in the queue appear to be from 'email@example.com' to address on the spammer domain. Does this mean that they are trying to get me to email them sensitive information from my server? At any rate, I need to figure out how to secure my server so that the only people who can send email from my server are authenticated domain users. Does anyone know the best way to lock this down so that only domain accounts can send email, and no one else? Users need to be able to send mail both in and out of the office. Remote email is done through Remote Web Workplace and ActiveSync to smartphones. I am using the Microsoft-Server-ActiveSync web application in my default web site in IIS 6, in case that is providing any security holes for spammers to get through.
It sounds like backscatter (NDR spam) to me, rather than your server being an open relay. Exchange Server 2003 by default is not an open relay. You can check whether or not your Exchange server is an open relay by using any number of online tests. You can help prevent backscatter spam in a few ways:
Disable NDR's on your Exchange server.
Filter email sent to recipients that don't exist in AD.
After reading the other answers and reading the comments to my answer, here's an updated answer:
Filter email sent to recipients that don't exist in AD. This is probably the more preferred method to deal with backscatter at your Exchange server.
Disable NDR's on your Exchange server. This is the least preferred method and was probably not the best suggestion on my part. While it does deal with the problem, it may not be completely in the spirit of RFC 5321 (although it probably is acceptable based on the verbage of RFC 5321 section 6.2).
The basic answer is you're suffering from a backscatter problem. The solution is to turn on recipient filtering as detailed in the MS KB article.
That's a short-term solution. The problem with recipient filtering is that it leaves you vulnerable to a directory harvest attack. If you want to really secure your server, you shouldn't accept SMTP connections directly. The best thing to do is use a service like Postini or MXLogic and only accept SMTP connections from those systems.