System Administration Guide: Resource Management and Network Services

Domains and Subdomains

Email addressing uses domains. A domain is a directory structure for network address naming. A domain can have one or more subdomains. The domain and subdomains of an address can be compared to the hierarchy of a file system. Just as a subdirectory is considered to be inside the directory above it, each subdomain in a mail address is considered to be inside the location to its right.

The following table shows some top-level domains.

Table 26-7 Top-Level Domains

Domain	Description
`com`	Commercial sites
`edu`	Educational sites
`gov`	United States government installations
`mil`	United States military installations
`net`	Networking organizations
`org`	Other nonprofit organizations

Domains are case insensitive. You can use uppercase, lowercase, or mixed-case letters in the domain part of an address without making any difference.

For more information about domains, refer to "Introduction to DNS (Overview)" in System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP).

Name Service Domain Name and Mail Domain Name

When you are working with name service domain names and mail domain names, remember the following.

By default, the sendmail program strips the first component from the NIS or NIS+ domain name to form the mail domain name. For example, if an NIS+ domain name were bldg5.example.com, its mail domain name would be example.com.
Although mail domain addresses are case insensitive, the NIS or NIS+ domain name is not. For the best results, use lowercase characters when setting up the mail and NIS or NIS+ domain names.
The DNS domain name and the mail domain name must be identical.

For more information, refer to "Interactions of sendmail With Name Services".

Typical Format for Mail Addresses

Typically, a mail address has the following format. For further details, refer to "Route-Independent Mail Addresses".

user@subdomain. ... .subdomain2.subdomain1.top-level-domain

The part of the address to the left of the @ sign is the local address. The local address can contain the following.

Information about routing with another mail transport (for example, bob::vmsvax@gateway or smallberries%mill.uucp@gateway)
An alias (for example, iggy.ignatz)

Note - The receiving mailer is responsible for determining what the local part of the address means. For information about mailers, refer to "Mailers".

The part of the address to the right of the @ sign shows the domain levels, which is where the local address resides. A dot separates each subdomain. The domain part of the address can be an organization, a physical area, or a geographic region. Furthermore, the order of domain information is hierarchical--the more local the subdomain, the closer it is to the @ sign.

Route-Independent Mail Addresses

Mail addresses can be route independent. Route-independent addressing requires the sender of an email message to specify the name of the recipient and the final destination. A high-speed network, such as the Internet, uses route-independent addresses. Route-independent addresses can have this format.

user@host.domain

Route-independent addresses for UUCP connections can have this address format.

host.domain!user

The increased popularity of the domain-hierarchical naming scheme for computers is making route-independent addresses more common. Actually, the most common route-independent address omits the host name and relies on the domain name service to properly identify the final destination of the email message.

user@domain

Route-independent addresses are read by searching for the @ sign and then reading the domain hierarchy from the right (the highest level) to the left (the most specific part of the address to the right of the @ sign).

Mailbox Files

A mailbox is a file that is the final destination for email messages. The name of the mailbox can be the user name or the identity of a specific function, such as the postmaster. Mailboxes are in the /var/mail/username file, which can exist either on the user's local system or on a remote mail server. In either instance, the mailbox is on the system to which the mail is delivered.

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. Do not use NFS-mounted file systems as the destination for a user's mailbox. Specifically, do not direct mail to a mail client that is mounting the /var/mail file system from a remote server. Mail for the user, in this instance, should be addressed to the mail server and not to the client host name. NFS-mounted file systems can cause problems with mail delivery and handling.

The /etc/mail/aliases file and name 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.

The following table shows some common naming conventions for special-purpose mailboxes.

Table 26-8 Conventions for the Format of Mailbox Names

Format	Description
username	User names are frequently the same as mailbox names.
Firstname`.`Lastname Firstname`_`Lastname Firstinitial`.`Lastname Firstinitial`_`Lastname	User names can be identified as full names with a dot (or an underscore) that separates the first and last names, or by a first initial with a dot (or an underscore) that separates the initial and the last name.
`postmaster`	Users can address questions and report problems with the mail system to the `postmaster` mailbox. Each site and domain should have a `postmaster` mailbox.
`MAILER-DAEMON`	`sendmail` automatically routes any mail that is addressed to the `MAILER-DAEMON` to the postmaster.
aliasname`-request`	Names that end in `-request` are administrative addresses for distribution lists. This address should redirect mail to the person who maintains the distribution list.
`owner-`aliasname	Names that begin with `owner-` are administrative addresses for distribution lists. This address should redirect mail to the person who handles mail errors.
`owner-owner`	This alias is used when no `owner-`aliasname alias exists for errors to be returned to. This address should redirect mail to the person who handles mail errors and should be defined on any system that maintains a large number of aliases.
local`%`domain	The percent sign (`%`) marks 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. Although many people use the `%` convention, it is not a formal standard. This convention is referred to as the "percent hack." This feature is often used to help debug mail problems.

Starting with sendmail version 8, the envelope sender for mail that is sent to a group alias has been changed to the address that is expanded from the owner alias, if an owner alias exists. This change enables any mail errors to be sent to the alias owner, rather than being returned to the sender. With this change, users notice that mail that was sent to an alias looks as if it came from the alias owner, when delivered. The following alias format helps with some of the problems that are associated with this change.

mygroup: :include:/pathname/mygroup.list
owner-mygroup: mygroup-request
mygroup-request: sandys, ignatz

In this example, the mygroup alias is the actual mail alias for the group. The owner-mygroup alias receives error messages. The mygroup-request alias should be used for administrative requests. This structure means that in mail sent to the mygroup alias, the envelope sender changes to mygroup-request.

Mail Aliases

An alias is an alternate name. For email, you can use aliases to assign a mailbox location or to define mailing lists. For a task map, refer to "Administering Mail Alias Files (Task Map)" in Chapter 25, Mail Services (Tasks). Also, you can refer to "Mail Alias Files" in this chapter.

For large sites, the mail alias typically defines the location of a mailbox. Providing a mail alias is like providing a room number as part of the address for an individual at a large corporation that occupies multiple rooms. If you do not provide the room number, the mail is delivered to a central address. Without a room number, extra effort is required to determine where within the building the mail is to be delivered, and the possibility of an error increases. For example, if two people who are named Kevin Smith are in the same building, only one of them might get mail. To correct the problem, each Kevin Smith should have a room number added to his address.

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

When you create alias entries, type one alias per line. You should have only one entry that contains the user's system name. For example, you could create the following entries for user ignatz.

ignatz: iggy.ignatz
iggyi: iggy.ignatz
iggy.ignatz: ignatz@mars

You can create an alias for local names or domains. For example, an alias entry for user fred, who has a mailbox on the system mars and is in the domain planets, could have this entry in the NIS+ aliases table.

fred: fred@planets

When you create mail lists that include users outside your domain, create the alias with the user name and the domain name. For example, if you have a user named smallberries on system privet, in domain example.com, create the alias as smallberries@example.com. The email address of the sender is now automatically translated to a fully qualified domain name when mail goes outside the user's domain.

The following list describes methods for creating and administering mail alias files.

You can create mail aliases for global use in the NIS+ mail_aliases table, the NIS aliases map, or in local /etc/mail/aliases files. You can also create and administer mailing lists that use the same alias files.
Depending on the configuration of your mail services, you can administer aliases by using the NIS or NIS+ name service to maintain a global aliases database or by updating all the local /etc/mail/aliases files to keep them synchronized.
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 local /etc/mail/aliases file, which anyone can use. Users cannot normally create or administer NIS or NIS+ alias files.


26. Mail Services (Reference) Software and Hardware Components of Mail Services Software Components Mail Addresses