The -bm command-line switch (the default) causes sendmail to run once in the foreground. A list of recipients is taken from the command line (unless the -t command-line switch is used), and the message is read from the standard input and delivered.

This is the mode MUAs use when they invoke sendmail on the user's behalf. The sendmail program processes the recipients first, then the message header, then the message body. Usually, the envelope recipients are those on the command line. But if the -t command-line switch is also used, the recipients are taken from the message header. The envelope sender is more difficult to determine:

• Trusted users, and programs running under the identity of those users, can specify the address of the sender by using the -f command-line switch^[7] (-f) when running sendmail. Trusted users are those that are declared with a T configuration command (Section 10.8.1.1). If anyone other than a trusted user uses the -f command-line switch, an X-Authentication-Warning: header (X-Authentication-Warning:) will be added to the message to show that the sender was changed by an unauthorized user.

  ^[7] The -r is a synonym for -f, but -r is deprecated, so we don't mention it directly.

• Otherwise, sendmail tries to use the user identity of the invoking program to determine the sender.

• When generating a mail bounce message, the sender becomes the name specified by the value of the $n sendmail macro ($n), usually mailer-daemon.