As distributed, sendmail(1M) supports the following types of addresses:

username

Each local username is listed in the local host's /etc/passwd file.

pathname

Messages addressed to the absolute pathname of a file are appended to that file.

|command

If the first character of the address is a vertical bar (|), sendmail(1M) pipes the message to the standard input of the command the bar precedes.

username@domain

If domain does not contain any `.' (dots), then it is interpreted as the name of a host in the current domain. Otherwise, the message is passed to a mailhost that determines how to get to the specified domain. Domains are divided into subdomains separated by dots, with the top-level domain on the right.

For example, the full address of John Smith could be:

js@jsmachine.Podunk-U.EDU

if he uses the machine named jsmachine at Podunk University.

... [host!] host!username

These are sometimes mistakenly referred to as ``Usenet'' addresses. uucp(1C) provides links to numerous sites throughout the world for the remote copying of files.

Other site-specific forms of addressing can be added by customizing the sendmail.cf configuration file. See sendmail(1M) for details. Standard addresses are recommended.

/etc/mail/aliases is formatted as a series of lines of the form

aliasname:address[, address]

aliasname is the name of the alias or alias group, and address is the address of a recipient in the group. Aliases can be nested. That is, an address can be the name of another alias group. Because of the way sendmail(1M) performs mapping from upper-case to lower-case, an address that is the name of another alias group must not contain any upper-case letters.

Lines beginning with white space are treated as continuation lines for the preceding alias. Lines beginning with # are comments.

An alias of the form:

owner-aliasname : address

sendmail directs error-messages resulting from mail to aliasname to address, instead of back to the person who sent the message. sendmail rewrites the SMTP envelope sender to match this, so owner-aliasname should always point to alias-request, and alias-request should point to the owner's actual address:

owner-aliasname:      aliasname-request
aliasname-request     address

An alias of the form:

aliasname: :include:pathname

with colons as shown, adds the recipients listed in the file pathname to the aliasname alias. This allows a private list to be maintained separately from the aliases file.

The aliases file on the master NIS server is used for the mail.aliases NIS map, which can be made available to every NIS client. The mail_aliases table serves the same purpose on a NIS+ server. Thus, the /etc/mail/aliases* files on the various hosts in a network will one day be obsolete. Domain-wide aliases should ultimately be resolved into usernames on specific hosts. For example, if the following were in the domain-wide alias file:

jsmith:js@jsmachine

then any NIS or NIS+ client could just mail to jsmith and not have to remember the machine and username for John Smith.

If a NIS or NIS+ alias does not resolve to an address with a specific host, then the name of the NIS or NIS+ domain is used. There should be an alias of the domain name for a host in this case.

For example, the alias:

jsmith:root

sends mail on a NIS or NIS+ client to root@podunk-u if the name of the NIS or NIS+ domain is podunk-u.

When an alias (or address) is resolved to the name of a user on the local host, sendmail(1M) checks for a ~/.forward file, owned by the intended recipient, in that user's home directory, and with universal read access. This file can contain one or more addresses or aliases as described above, each of which is sent a copy of the user's mail.

Care must be taken to avoid creating addressing loops in the ~/.forward file. When forwarding mail between machines, be sure that the destination machine does not return the mail to the sender through the operation of any NIS aliases. Otherwise, copies of the message may "bounce." Usually, the solution is to change the NIS alias to direct mail to the proper destination.

A backslash before a username inhibits further aliasing. For instance, to invoke the vacation program, user js creates a ~/.forward file that contains the line:

\js, "|/usr/ucb/vacation js"

so that one copy of the message is sent to the user, and another is piped into the vacation program.