Replace your mailbanner (the text one sees when telnetting to your port 25) with something meaningless.
This is what you see when you do telnet to port 25.
nikesh@poison:~> telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 poison.hell.com ESMTP Postfix
Now to remove this banner incformation, you need to change a variable setting in your main.cf file, something like ...
smtpd_banner = $myhostname ESMTP Sorry, No banner
After doing the change, restart your postfix server and try doing telnet again to port 25
nikesh@poison:~> telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 poison.hell.com ESMTP Sorry, No banner
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source:http://linuxpoison.blogspot.com/2008/05/13578175809031.html