When I was running Sendmail on a CentOS 5 server, I needed to use the m4 command after modifying
/etc/sendmail.mc to rebuild /etc/sendmail.cf from
that file, i.e., m4 /etc/mail/sendmail.mc >
/etc/mail/sendmail.cf. E.g., see
Configuring Sendmail to
Use a Smart Host, but that is no longer needed with CentOS 7 and,
though I went from CentOS 5 to 7, may have gone away as a needed step
in CentOS 6. Now, if you modify /etc/sendmail.mc, when
you restart sendmail, e.g., with service sendmail restart,
the sendmail.cf file is automatically rebuilt.
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