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)).
