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If you are still wanting to receive incoming mail, you can't stop someone from telneting to the SMTP port. As long as the proper SMTP commands are sent, here is no way for the mail daemon to differentiate between a remote mail client or telnet.
If you want to block incoming mail or at least limit the IP addresses that can send you mail, then that is a different situation.
Originally posted by yenonn what if someone telnet port 25 and make a virus attachment through the mail? could it be possible make an attachment through telnetting?
Yes. As already stated, when you "telnet" to port 25, you are acting as another SMTP server attempting to deliver mail to your server.
Originally posted by yenonn then, i can simply impersonate a particular to sending email on behalf of him???? is that true?
Yes. Have you never recieved spam before?
Originally posted by yenonn then, what is the protection ?
Protection against what? Imperonating someone else's email? There are no good solutions really. Some people will say that SPF provides good protection, but it doesn't. Most SMTP servers these days provide you with methods to reject mail to unknown users. But spam is simply a fact of life if you are going to run an SMTP server on today's internet.
it will limit the risk to a degree because people wont be able to send anything using your SMTP server without the auth user/pass but people will still be able to telnet in and attempt, no way of stopping that. Do you recieve emails on this box? or just sending? If you are just sending you may be able to restrict the access to the SMTP server to IP's you know.