If you are writing an SMTP agent, you might find it instructive to learn to speak SMTP yourself, both to aid debugging and to get a feel for what is happening "under the hood" when Exchange receives a message from the internet. This is a lot easier than it might sound. Open the Run dialog box (press Window + R) and enter "telnet localhost smtp" (or substitute the name of your Gateway server in place of localhost). This will open a window where you can talk with the server. The server will begin the conversation by sending a banner, such as: