Despite its oft-documented insecurities, telnet remains a useful tool in the server administrator's toolkit. Linux Planet explains shows how to test basic SMTP, IMAP, HTTP, and IRC server functions using the venerable networking protocol.
Telnet is one of the older ways of doing business over a network: a network protocol running over TCP/IP, which allows a client to talk to a server remotely. Back in the day (such as when I first got online), telnet was the usual means to connect to a remote computer to get a console, and from there do server administration, check email, and run applications. These days ssh is the default for security reasons, but telnet still has its uses. Because telnet transmits all data as-is you can use it to open a raw TCP session, then talk to a server running a network service, and do a little debugging. At the very least, telnetting to a particular port on a server can confirm that you can reach the remote server from your machine, and that there is a running service bound to the port.
It's important to remember that telnet is NOT encrypted, so it's very vulnerable to packet-sniffing and man-in-the-middle attacks. You should never use it to transmit a username and password except for controlled tests using disposable accounts.
SMTP servers run on port 25, and you can use telnet to talk to them directly: