mail: cannot send message: process exited with a non-zero status

I have get the same problem in an Ubuntu 14.04 server. And I find error message in /var/log/mail.err, which said:

postfix/sendmail[27115]: fatal: open /etc/postfix/main.cf: No such file or directory

Then I just reconfigured postfix and solved this problem.

sudo dpkg-reconfigure postfix