System Administration Guide, Volume 3

Mail Services Terminology

In addition to the mail files and programs, many other components are required to establish a mail service. The following sections define these components and some of the terminology used to describe them.

The first section defines the terminology used when discussing the software parts of the mail delivery system. The next section focuses on the functions of the hardware systems in a mail configuration.

Mail Services Software Terminology

This section describes the software components of a mail system. Each service includes at least one of each of the following:

Mail user agent
Mail transfer agent
Mail delivery agent

Other software components include domain names, mail addresses, mailboxes, and mail aliases.

Mail User Agent

The mail user agent is the program that acts as the interface between the user and mail transfer agent, such as the sendmail program. The mail user agents supplied with the Solaris operating environment are /usr/bin/mail, /usr/bin/mailx, $OPENWINHOME/bin/mailtool, and /usr/dt/bin/dtmail.

Mail Transfer Agent

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

Accepts messages from the mail user agent
Resolves destination addresses
Selects a proper delivery agent to deliver the mail
Receives incoming mail from other mail transfer agents

Mail Delivery Agent

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

The UUCP mail delivery agent, which uses uux to deliver mail
The local mail delivery agent, which is mail.local in the standard Solaris release

Mailers

A mailer is a sendmail specific term. You can customize a mail delivery agent. A mailer is used by sendmail to identify a specific instance of a customized mail delivery agent or a mail transfer agent.

You need to specify at least one mailer in the sendmail.cf file of all systems in your network.

The smtp mailer uses SMTP to transfer a message. SMTP is the standard mail protocol used on the Internet. This is an example of an SMTP mail header:

To: paul@phoenix.stateu.edu
From: Iggy.Ignatz@eng.acme.com

If mail is sent between two users in the same domain, the header looks like this:

To: Irving.Who@eng.acme.com
From: Iggy.Ignatz@eng.acme.com

Use SMTP for sending mail outside your domain, especially for mailboxes that you must reach through the Internet.

The uucp-old mailer uses uux to deliver messages, but it formats headers with a domain-style address, and the To: and Cc: lines are formatted by domain, much like the SMTP headers. The uucp headers look like this:

To: paul@phoenix.stateu.com
From: ignatz@eng.acme.com

Use uucp-uudom for UUCP mail to systems that can handle and resolve domain-style names. The sender also must be able to handle domain-style names and be able to receive replies from the Internet.

The uucp-old mailer uses an exclamation point address in the headers. This is one of the original mailers. The headers look like this:

To: edu!stateu!phoenix!paul
From: acme!ignatz

You can define other mail delivery agents by providing a mailer specification in the sendmail.cf file. Additional information about mailers can be found in /usr/lib/mail/README.

Domain Names

A domain is a directory structure for network address naming. Electronic-mail addressing also uses domains. An email address has this format:

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 information about:

Routing with another mail transport (for example, bob::vmsvax@gateway or smallberries%mill.uucp@gateway)

An alias (for example, iggy.ignatz)

The receiving mailer is responsible for determining what the local part of the address means.

The part of the address to the right of the @ sign shows the domain address where the local address is located. A dot separates each part of the domain address. The domain can be an organization, a physical area, or a geographic region.

Domain addresses are case insensitive. It makes no difference whether you use uppercase, lowercase, or mixed-case letters in the domain part of an address.

The order of domain information is hierarchical--the more local the address, the closer it is to the @ sign.

The larger the number of subdomains, the more detailed the information that is provided about the destination. Just as a subdirectory in a file-system hierarchy is considered to be inside the directory above, each subdomain in the mail address is considered to be inside the location to its right.

The following table shows the top-level domains.

Table 35-3 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 organizations

!%@:: A Directory of Electronic Mail Addressing and Networks by Donnalyn Frey and Rick Adams (O'Reilly & Associates, Inc., 1993) contains a complete list of international top-level domain addresses; it is updated periodically.

For mail delivery, the name space domain name and the mail domain name occasionally do not match. However, the DNS domain name and the mail domain name must be identical. By default, the sendmail program strips the first component from the domain name to form the mail domain name. For example, if a NIS+ domain name were bldg5.eng.acme.com, its mail domain name would be eng.acme.com.

Note -

Although mail domain addresses are case insensitive, the name space domain name is not. For best results use lowercase characters when setting up the mail and name space domain names.

Mail Address

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: login names uniquely identify users.

When, however, you are administering a mail system that has more than one system with mailboxes, one or more domains, or when you have a UUCP (or other) mail connection to the outside world, mail addressing becomes more complex. Mail addresses can be route independent, route based, or a mixture of the two. Route-based addressing is based on old specifications and is not required or desired in most situations.

Route-Independent Addressing

Route-independent addressing requires the sender of an email message to specify the name of the recipient and the final destination address. Route-independent addresses usually indicate the use of a high-speed network like the Internet. In addition, newer UUCP connections frequently use domain-style names. Route-independent addresses can have this format:

user@host.domain

UUCP connections can use the following address format:

host.domain!user

The increased popularity of the domain hierarchical naming scheme for computers is making route-independent addresses more common. In fact, 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, then reading the domain hierarchy from the right (the highest level) to the left (the most specific address to the right of the @ sign).

Route-Based Addressing

Route-based addressing requires the sender of an email message to specify the local address (typically, a user name) and its final destination, as well as the route that the message must take to reach its final destination. Route-based addresses were fairly common on UUCP networks, and have this format:

path!host!user

Whenever you see an exclamation point as part of an email address, all (or some) of the route was specified by the sender. Route-based addresses are always read from left to right.

For example, an email address that looks like this:

venus!acme!sierra!ignatz

means that mail sent to the user named ignatz is first sent to the system named venus, next to acme, and then to sierra. (This is an example and not an actual route.) If any of the mail handlers is down, the message will be delayed or returned as undeliverable.

Mail sent through the uucp mailer is not restricted to using route-based addressing. Some uucp mailers also handle route-independent addressing.

Mailbox

A mailbox is a file on a mail server that is the final destination for email messages. The name of the mailbox can be the user name or a place to put mail for someone with a specific function, like 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 case, 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 case, 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. Clients that NFS mount /var/mail go into "remote mode" and should arrange to have the server send and receive mail for them.

The /etc/mail/aliases file and name services like 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 35-4 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) 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`	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 addressed to the `MAILER-DAEMON` to the postmaster.
`aliasname-request`	Names ending in `-request` are administrative addresses for distribution lists. This address should redirect mail to the person who maintains the distribution list.
`owner-aliasname`	Names beginning 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 there is no `owner-aliasname` alias 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. It is referred to as the "percent hack." This feature is often used to help debug mail problems.

Starting with version 8, the envelope sender for mail sent to a group alias is changed to the address expanded from the owner alias, if an owner alias exists. This change allows for any mail errors to be sent to the alias owner rather than being returned to the sender. What users will notice is that mail they send to an alias, when delivered, will look like it came from the alias owner. The following alias format will help with some of the problems 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; and 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 electronic mail, you can use aliases to assign a mailbox location or to define mailing lists.

For large sites, the mail alias typically defines the location of a mailbox. 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 within the building the mail is to be delivered, and the possibility of error increases. For example, if two people named Kevin Smith are in the same building, only one of them will get mail.

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 named ignatz on system mars, in domain eng.acme.com, create the alias ignatz@eng instead of ignatz@mars. If user ignatz changes the name of his system but remains within the engineering domain, you do not need to update alias files to reflect the change in system name.

When creating 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 who is in the domain planets could have this entry in the NIS+ aliases table:

fred: fred@planets

When creating 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 mgmt.acme.com, create the alias as smallberries@mgmt.acme.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.

Uses for Aliases Files

You 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 using 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 can be used by anyone. Users cannot normally create or administer NIS or NIS+ alias files.

Hardware Components of a Mail Configuration

A mail configuration requires three elements, which can be combined on the same system or provided by separate systems:

A mail host
At least one mail server
Mail clients

When you want users to communicate with networks outside your domain, you must also add a fourth element, a mail gateway. The following sections describe each hardware component.

Mail Host

A mail host is the machine that you designate as the main mail machine on your network. It is the machine to which other systems at the site forward mail that they cannot deliver. You designate a system as a mail host in the hosts database by adding the word mailhost to the right of the IP address in the local /etc/hosts file or in the hosts file in the name service. You must also use the main.cf file as the mail-configuration file on the mail host system.

A good candidate for mail host is a system on the local area network that also has a modem for setting up PPP or UUCP links over telephone lines. Another good candidate is a system configured as a router from your network to the Internet global network. (See Chapter 21, Overview of PPP, Chapter 25, Overview of UUCP, and "Configuring Routers" for more information.) If none of the systems on your local network has a modem, designate one as the mail host.

Some sites use standalone machines that are not networked in a time-sharing configuration; that is, the standalone machine serves terminals attached to its serial ports. You can set up electronic mail for this configuration by treating the standalone system as the mail host of a one-system network.

Mail Server

A mailbox is a single file that contains email for a particular user. Mail is delivered to the system where the user's mailbox resides: the local machine or a remote server. A mail server is any system that maintains user mailboxes in its /var/mail directory.

The mail server routes all mail from a client. When a client sends mail, the mail server puts it in a queue for delivery. After the mail is in the queue, a user can reboot or turn off the client without losing those mail messages. When the recipient gets mail from a client, the path in the "From " line of the message contains the name of the mail server. If the recipient responds, the response goes to the user's mailbox. Good candidates for mail servers are systems that provide a home directory for users or that are backed up regularly.

If the mail server is not the user's local system, users in configurations using NFS software can mount the /var/mail directory by using the /etc/vfstab file (if they have root access) or by using the automounter. If NFS support is not available, the users can log in to the server to read their mail.

If users on your network send other types of mail, such as PostScript^TM files, audio files, or files from desktop publishing systems, you need to allocate more space on the mail server for mailboxes.

One advantage to establishing a mail server for all mailboxes is that it makes backups easy. Having mail spread over many systems makes it hard to do backups. The disadvantage of storing many mailboxes on one server is that the server can be a single point of failure for many users, but the advantages of providing good backups usually make the risk worthwhile.

Mail Client

A mail client is any system that receives mail on a mail server and does not have a local /var/mail directory. This is known as remote mode. Remote mode is enabled by default in /etc/mail/subsidiary.cf.

You must check that the mail client has the appropriate entry in the /etc/vfstab file and a mount point to mount the mailbox from the mail server. Also make sure that the alias for the client is directed to the mail server's host name, not to the client's.

Mail Gateway

The mail gateway is a machine that handles connections between networks running different communications protocols or communications between different networks using the same protocol. For example, a mail gateway might connect a TCP/IP network to a network running the Systems Network Architecture (SNA) protocol suite.

The simplest mail gateway to set up is one that connects two networks that use the same protocol or mailer. This system handles mail with an address for which sendmail cannot find a recipient in your domain. If a mail gateway exists, sendmail uses it for sending and receiving mail outside your domain.

You can set up a mail gateway between two networks using unmatched mailers, as shown in the next figure. To support this, you must customize the sendmail.cf file on the mail gateway system, which can be a difficult and time-consuming process.

Figure 35-1 Gateway Between Different Communications Protocols

If you have to set up a mail gateway, you should find a gateway-configuration file that is close to what you need and modify it to fit your situation.

If you have a machine providing connections to the Internet, you can configure that machine as the mail gateway. Carefully consider your site's security needs before you configure a mail gateway. You might need to create a firewall gateway between your corporate network and the outside world, and set that up as the mail gateway.