There are four ways that email messages can be submitted to sendmail by other programs. One way is with the -t command-line switch (-t). This causes sendmail to read the message on its standard input, and to parse the addresses from the header lines. Another way is with the -bs command-line switch (-bs), which causes sendmail to speak SMTP on its standard input and output. The third way is to specify recipients on the command line, and to feed sendmail the message on its standard input. The fourth way is to connect directly to sendmail's MSA port (FEATURE(no_default_msa)).

For the -t and -bs forms of submission to behave like messages submitted via the MSA port, it is desirable to use a special configuration file. This -A command-line switch does just that. By following it with a c character, you tell sendmail to use a configuration file named submit.cf in place of the default configuration file. If the -A switch is followed by an m character, the default configuration file is used:

% /usr/sbin/sendmail -Ac               use submit.cf 
% /usr/sbin/sendmail -Am               use sendmail.cf 

If the -A switch is omitted, the choice of configuration file depends on the mode under which sendmail was run. That is, if it was run with a -t or a -bs, the behavior is that of -Ac. Otherwise, the behavior is that of -Am.

Note that this -A command-line switch can be used by ordinary users without causing sendmail to drop any special privileges.

The submit.cf file is installed automatically when you install sendmail.cf (Section 2.6.2.4). A custom one can easily be created using the msp mc feature (FEATURE(msp)).