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Your mail server isn’t working fine. When you connect to it using telnet, you should see a welcome message along the lines of:
220 your.server.name ESMTP Postfix
(You can check the greeting that you should be seeing by running postconf smtpd_banner.)
You don’t get that, so the mail server isn’t running properly. send_mail is probably hanging waiting for that initial message.
Restart Postfix, and look in the /var/log/mail.* log files; there may be a clue in there as to why it’s not working.
Source:stackexchange.com