-bd command-line switch causes
sendmail to become a daemon, running in the
background, listening for and handling incoming SMTP
To become a daemon, sendmail first performs a
fork(2). The parent then exits, and the child
becomes the daemon by disconnecting itself from its controlling
terminal. The -bD command-line switch can be used
to prevent the fork(2) and the detachment and
allows the sendmail program's
behavior to be observed while it runs in daemon mode.
As a daemon, sendmail does a
listen(2) on TCP port 25 by default for incoming
SMTP messages. When another site
connects to the listening daemon, the daemon performs a
fork(2), and the child handles receipt of the
incoming mail message.