Installing and Administering Internet Services

174 Chapter4
Installing and Administering sendmail
How sendmail Works
files.) Mail to programs is normally piped to the prog mailer
(/usr/bin/sh -c), which executes the command specified in the alias or
.forward file definition. (You can restrict the programs that can be run
through the aliases or .forward files. See “Security” on page 190 for
more information.) Mail to a file is directly appended to the file by
sendmail if certain conditions of ownership and permission are met.
After all alias expansion is complete, mail that is addressed to a local
user name is routed to the local mailer (/usr/bin/rmail), which
deposits the message in the user’s mailbox.
The Default Routing Configuration
The installed configuration file, if unmodified, routes mail depending on
the syntax of the recipient addresses as described in the following
sections.
Local Addresses The following forms are recognized as local addresses
and are delivered locally:
user
user @ localhost
user @ localhost . localdomain
user @ alias
user @ alias . localdomain
user @[ local_host’s_internet_address ]
localhost ! user
localhost ! localhost ! user
user @ localhost .uucp
UUCP Addresses Where host is not the local host name, addresses of
the following forms are recognized as UUCP addresses:
host ! user
host ! host ! user
user @ host . uucp
If your host has a direct UUCP connection to the next host in the path,
the mail is delivered to that host through UUCP. If not, the message is
returned with an error. The supplied configuration file provides detailed