I don't think you've really understood what it means to use telnet to connect to postfix. Telnet is just used as a very basic network application which allows direct interaction with something at the other end of a TCP connection. you are not literally "telnetting" to it, despite the language used, as it is not the telnet protocol being used, just basic TCP. You can't block the use of the telnet application itself as there is nothing within SMTP to allow the mail client to identify what it is - thunderbird, outlook, mutt, telnet... all say the same things over SMTP.
If you forget about the telnet side completely, if you want to restrict access you can...
- use ssl / tls instead to encrypt the connection (telnet connections will still work just super if used over a ssl tunnel application like stunnel)
- restrict connections to certain IP addresses (again telnet is still completely possible from those permitted sources but no connectivity at all is allowed from other addresses)
there is *nothing* wrong with using telnet to debug things, it is not evil, and if you think it is, then you're mistaken.
Last edited by acid_kewpie; 01-29-2009 at 01:23 PM.