Are you running an open relay? (Windows)

If your SMTP mail server is set up so it will process email that isn’t being sent to or from a local user, it's an open relay. Many organizations are running open relays without even knowing it. What's the problem? Spammers can route their spam messages through your server to disguise their identities. That can cost you money and lost productivity when your network's resources are being used (for no charge) by someone else. It can also cause your domain to end up on spam blacklists, which means your legitimate mail may not get through to recipients.

Don't know whether your mail server is running as an open relay? There are a number of websites that make it easy to test your server. One such site is run by the Network Abuse Clearinghouse at www.abuse.net/relay.html.