System Administration Guide: Network Services

Software Components

Each mail service includes at least one of each of the following software components.

This section also describes these software components.

Mail User Agent

The mail user agent is the program that acts as the interface between the user and mail transfer agent. The sendmail program is a mail transfer agent. The Solaris operating system supplies the following mail user agents.

Mail Transfer Agent

The mail transfer agent is responsible for the routing of mail messages and the resolution of mail addresses. This agent is also known as a mail transport agent. The transfer agent for the Solaris operating system is sendmail. The transfer agent performs these functions.

Local Delivery Agent

A local delivery agent is a program that implements a mail delivery protocol. The following local delivery agents are provided with the Solaris operating system.

Changes From Version 8.12 of sendmail provides information on these related topics.

Mailers and sendmail



Simple Mail Transfer Protocol (SMTP) Mailers


UNIX-to-UNIX Copy Program (UUCP) Mailers


UUCP defines these mailers.










Note –


Mail Addresses

The mail address contains the name of the recipient and the system to which the mail message is delivered. When you administer a small mail system that does not use a name service, addressing mail is easy. The login names uniquely identify the users. Complexity is introduced if you are administering a mail system that has more than one system with mailboxes or that has one or more domains. Additional complexity can be generated if you have a UUCP (or other) mail connection to servers outside your network. The information in the following sections can help you understand the parts and complexities of a mail address.

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 14–7 Top-Level Domains




Commercial sites 


Educational sites 


United States government installations 


United States military installations 


Networking organizations 


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 errors.

Name Service Domain Name and Mail Domain Name

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

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.

Note –

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

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, so the more local the subdomain, the closer the subdomain 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.


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


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.


Route-independent addresses are first read by searching for the @ sign. The domain hierarchy is then read 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, 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 14–8 Conventions for the Format of Mailbox Names




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. Alternately, user names can be identified by a first initial with a dot (or an underscore) that separates the initial and the last name. 


Users can address questions and report problems with the mail system to the postmaster mailbox. Each site and domain should have a postmaster mailbox.


sendmail automatically routes any mail that is addressed to the MAILER-DAEMON to the postmaster.


Names that end in -request are administrative addresses for distribution lists. This address should redirect mail to the person who maintains the distribution list.


Names that begin with owner- are administrative addresses for distribution lists. This address should redirect mail to the person who handles mail errors.


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. This address also should be defined on any system that maintains a large number of aliases.


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, this convention 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 the mail 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 13, 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. So, 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 who is 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.