An alternative configuration file can be specified using the -C
flag; for example:
/usr/lib/sendmail -Ctest.cf -oQ/tmp/mqueue
This uses the configuration file test.cf instead of the default /etc/sendmail.cf.. If the -C flag has no value it defaults to sendmail.cf in the current directory.
sendmail(1M) gives up its setuid root permissions when you use this flag, so it is common to use a publicly writable directory (such as /tmp) as the spool directory (QueueDirectory or Q option) while testing.