commandline session $nc 127.0.0.1 25 220 debian.Jeff ESMTP Exim 4.80 Thu, 27 Jun 2013 00:05:20 +0530 ^C $nc gnu.org no port[s] to connect to $nc gnu.org 25 ^C $nc -g gnu.org 25 no port[s] to connect to $nc -g gnu.org gnu.org 25 ^C $nc -g 127.0.0.1 127.0.0.1 25 ^C $nc www.youtube.com no port[s] to connect …