Microsoft Knowledge Base Article 153119 contains the following summary:
This article describes how to telnet to port 25 on a computer that runs Simple Mail Transfer Protocol (SMTP) services to troubleshoot SMTP communication problems. By default, SMTP listens on port 25.
You can use the following troubleshooting steps that are appropriate for the type of problem that you are experiencing. For example, if you are having problems sending over SMTP between two of your Microsoft Exchange 2000 Server servers, you can test the SMTP connectivity by using Telnet on the sending server to connect to port 25 on the destination server. Alternatively, if you are having problems receiving SMTP mail from the Internet, you can follow the steps that are listed in this article to test connectivity to your SMTP server from a host that resides on the Internet and that is not on your network.