How can I monitor SMTP traffic coming from all machines?

Windows NT and 2000 Server come with a built-in Network Monitor application that will allow you to capture and analyze network packets based on specific filter criteria. However, the native utility will only analyze packets that are sent to or from the local computer. To perform analysis on an enterprise network you will need an additional utility such as Systems Management Server or another third-party application - HP Openview, Unicenter TNG or the like.

If your client is already using a firewall, I wouldn't be surprised if the firewall itself possessed some description of monitoring software that might potentially meet your needs in this situation. It's certainly worth investigating in the interests of saving money and not having to re-invent the wheel.