Deliver E-Mail on the local machine, and invoke a notification agent.
Reads configuration from /etc/hailbox.conf
Reads the message from standard input.

Usage: hailbox ADDRESS
               --check | -c
               --help | -h
               --version | -v

Hailbox delivers to a user's mailbox in a traditional UNIX mail spool.
The user's address may contain a domain part, but as Hailbox performs
only local delivery, any domain part is ignored.  The user part of the
address must identify a local mailbox.  Hailbox assumes that the mail
transfer agent has already prepended an appropriate From_ line to the
message, and it does not add one.  The user's mailbox is locked during
delivery using a POSIX file lock and a dot lock.  The exit status
takes the value of one of the constants in <sysexits.h> if delivery is
not successful.

When delivery is complete, Hailbox passes the user part of the address
through a filter.  The filtered address is used to invoke a
notification agent named in the configuration file.

Execute with a parameter of "--check" to check the validity of the
configuration file.

Examples: hailbox --check
          hailbox kodos@uranus.net << EoF
          From kang@uranus.net Fri Dec 31 23:59:59 9999
          Date:    Fri, 31 Dec 9999 23:59:59 -0000 (UTC)
          From:    Kang <kang@uranus.net>
          To:      Kodos <kodos@uranus.net>
          Subject: Invasion Plans

          We will use a large board with a nail in it.