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A mailbox is a directory on a mail server that is the final destination for email messages. The name of the mailbox may be the username or a place to put mail for someone with a specific function, such as the postmaster. Mailboxes can be in the /var/mail/username directory on the user's local system or on a mail server.

Mail should always be delivered to a local file system so that the user agent can pull mail from the mail spool and store it readily in the local mailbox.

CAUTION! Do not use NFS-mounted file systems as the destination for a user's mailbox. NFS-mounted file systems cause problems with mail delivery and handling if the server fails.

The Aliases database, the /etc/mail/aliases file, and naming services such as NIS and NIS+ provide mechanisms for creating aliases for electronic mail addresses so that users do not need to know the precise local name of a user's mailbox. Mail aliases provide aliases for names to the left of the @ sign. Because DNS provides aliases only for systems and domains (for names to the right of the @ sign), you cannot use DNS to maintain user or mailing list aliases.

Some common naming conventions for special-purpose mailboxes are shown in Table 1-5.

Table 1-5 Conventions for the Format of Mailbox Names

Format Description
username Usernames are frequently the same as mailbox names.
Firstname.Lastname, Firstname_Lastname, Firstinitial.Lastname, or Firstinitial_Lastname Usernames may be identified as full names with a dot (or an underscore) separating the first and last names or by a first initial with a dot (or an underscore) separating the initial and the last name.
postmaster Each site and domain is required by Internet standards to have a postmaster mailbox. Users can address questions and report problems with the mail system to the postmaster mailbox.
MAILER-DAEMON Any mail addressed to the MAILER-DAEMON is automatically routed to the postmaster by sendmail.
x-interest Names with dashes are likely to be a distribution list or a mailing list. This format is commonly used for net mail groups.
x-interest-request Names ending in -request are administrative addresses for distribution lists.
owner-x-interest Names beginning with owner- are administrative addresses for distribution lists.
local%domain The percent sign (%) shows a local address that is expanded when the message arrives at its destination. Most mail systems interpret mailbox names with % characters as full mail addresses. The % is replaced with an @ and the mail is redirected accordingly. Note that although many people use the % convention, it is not a formal standard. In the email community, it is referred to as the % hack.
usenet Required by Internet standards for any domain or system that processes or feeds UseNet news.


An alias is an alternative name. For electronic mail, you can use aliases to assign additional names to a user, route mail to a particular system, or define mailing lists.

You can create a mail alias for each user at your site to indicate where the mail is stored. Providing a mail alias is like providing a mail stop as part of the address for an individual at a large corporation. If you do not provide the mail stop, the mail is delivered to a central address. Extra effort is required to determine where the mail is to be delivered within the building, and the possibility of error increases. For example, if there are two people named Kevin Smith in the same building, the probability is high that each Kevin will receive mail intended for the other.

Use domains and location-independent addresses as much as possible when you create alias files. To enhance the portability and flexibility of alias files, make your alias entries as generic and system-independent as possible. For example, if you have a user named ignatz on a system oak in domain, create the alias as ignatz instead of ignatz@Eng or ignatz@oak. If the user ignatz changes the name of the system but remains within the engineering domain, you do not need to update any alias files to reflect the change in the system name.

When creating aliases that include users outside of your domain, create the alias with the username and the domain name. For example, if you have a user named smallberries on system privet in domain, create the alias as smallberries@Corp.

NOTE:  You can set an option in the file to translate the email address to a fully qualified domain name--a domain name that contains all of the elements needed to specify where an electronic mail message should be delivered or where an NIS+ table is located—when mail goes outside of the user's domain. See Chapter 4 for more information.

Uses for Alias Files

You create mail aliases for global use in the NIS+ mail_aliases table, in the NIS aliases map, or, if your site does not use a naming service, in local /etc/mail/aliases files. You can also create and administer mailing lists using the same alias files.

Depending on the configuration of your mail services, you can administer aliases by using the NIS or NIS+ naming service to maintain a global aliases database or by updating all of the local /etc/mail/aliases files to keep them in sync. See Chapter 3, "Setting Up and Administering Mail Services," for information on how to create aliases.

Users can also create and use aliases. They can create aliases either in their local .mailrc file, which only they can use, or in their system's local /etc/mail/aliases file, which can be used by anyone. Users cannot create or administer NIS or NIS+ alias files. Users cannot administer the local /etc/mail/aliases file unless they have access to the root password on their system.

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