Telnet is an extremely useful tool for troubleshooting issues related to SMTP and mail flow. For example, you can use telnet to:

• Verify that SMTP is installed properly, and that it has all the necessary commands.

• Ensure that your server is accessible over the Internet.

• Attempt mail delivery directly over the TCP port.

• Determine that all servers are accepting connections.

• Determine if a firewall is blocking a connection.

• Ensure that a single user can receive mail.

• Ensure that a specific domain can receive mail.

• Ensure that a specific user or domain can send mail to your domain.