i have a mail server in my machine and is it possible to hack into the box telnetting at port 25?

Telnet is just a generic utility used to access the port. Whether or not your mail server is vulnerable to a hack attempt really depends on the configuration, patches, etc.

thanks for the reply stick man, can u tell me more on this. is it possible to give linux command to remote host once it accepts telnet at port 25

Servers such as your mail one accept connections from other programs or servers, and communicate data such as emails. This data is normally ASCII/text based, and so human readable. Telnet allows you to connect to a server and pass text commands, so you can issue your own GET requests for web pages or send email details.

If the server you are running has vulnerabilities such as buffer overflows, then by sending a maliciously crafted lot of text you can cause the server program to act incorrectly, maybe executing code or whatever it's contorted to do.

The port and the connecting program aren't important really, it's the security of the code that was written for the server's input checking.

Adding to what Proud said, the telnet program is just a client that allows connections to arbitrary TCP ports. Depending on what service is runing on that port, it may be possible to issue commands in clear text simply by typing. A number of common services are usable this way, such as HTTP, SMTP, POP3, etc... The ability to use the telnet program to connect to say, port 25/TCP (usually SMTP) is no different from what happens when Evolution, Kmail, Thunderbird, etc would connect to port 25/TCP.

The reason why telnet causes frequent confusion is that there is also a telnet service that is run by telnetd (the Telnet service daemon). The telnet service allows remote connections to authenticate as a local user and gain a shell. Telnet itself usually isn't vulnerable to attacks, since the protocol is fairly simple. The reason telnet service gets a bad reputation is because it passes everything "in the clear", i.e. not in an encrypted fashion. This lets potential snoopers view the actual login username and password being used. That's why use of the telnet service (i.e. telnetd, usually run on port 23/TCP) is highly discouraged and SSH is the recommended remote login protocol.