When I was running Sendmail on a CentOS 5 server, I needed to use the m4 command after modifying
/etc/sendmail.cffrom 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.cffile is automatically rebuilt.
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