helo command "500 Unrecognised command" from remote machine

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http://www.linuxquestions.org – When I telnet to my mail server(mailserver.mydomain.com) from mymachine.mydomain.com, I get a "500 Unrecognised command" error from helo command. But if I ssh to the mail server(mailserver.mydomain.com) and then telnet locally, helo command works fine. Any ideas what's causing this behavior? Example below. from mymachine.mydomain.com. Code: $ telnet mailserver.mydomain.com 25 Trying xx.xx.xx.xx... Connected to mailserver.mydomain.com. Escape character is '^]'. 220 mailserver.mydomain.com ESMTP; Thu, 06 Jun 2013 11:33:55 -0700 (PDT) helo machine.mydomain.com 500 Unre (HowTos)