The -C flag directs sendmail to use an alternate configuration file. For example, the following line directs sendmail to use the test.cf file instead of the default /usr/lib/sendmail.cf file:
/usr/lib/sendmail -Ctest.cf
If the -C flag appears without a filename, sendmail uses the file sendmail.cf in the current directory. Thus, the -C flag directs sendmail to ignore any /usr/lib/sendmail.fc ("frozen") file that may be present.