How to troubleshoot SMTP communication problems using telnet

To open a command line, Click Start, click Run, type cmd in the Open box, and then click OK. You can start a Telnet session by using the Telnet command in the following format:

Note Press ENTER after you type each line.

telnet servername portnumber

For example, type:

telnet mail.contoso.com 25

Note You can replace servername with the IP address or the FQDN of the SMTP server that you want to connect to.

More details from support.microsoft.com/kb/153119.