The initial behavior of sendmail is determined largely by the command line used to invoke it. The command line can, for example, cause sendmail to use a different configuration file or to rebuild the aliases file rather than deliver mail. The command line can be typed at your keyboard, executed from a boot-time script, or even executed by an MUA when sending mail.
The format of the sendmail command line is:
argv switches recipients
argv is the name used to run
switches, if any are present, must always
precede the list of recipients. The
recipients is a list of zero or more
recipient address expressions.