I had this problem and the
This error happened to me because of there was the “Address already in use”. In my case I just restarted the VPS. In other cases is hanging waiting for a message. For this reason remember to put the body inside the mail command like this:
$ echo "Test mail from postfix" | mail -s "Test Postfix" email@example.com
For other mail problems, take a look to /var/log/mail.log
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