Mailers

A mailer is a protocol that specifies the policy and mechanics used by sendmail when it delivers mail. You need to specify a mailer in the sendmail.cf file of a relay host or a gateway. The mailer for a relay host must match the mailer on the system outside of your domain. A gateway is a more complicated relay host (alternatively, you can think of a relay host as a simple gateway) that can communicate with more than one type of mailer.

The mailers provided with SunOS 5.x system software are as follows:

•  smartuucp (the default relay mailer) uses uux (UNIX-to-UNIX command execution) to deliver messages but formats headers with a domain-style address. The To: and CC: lines are formatted by domain. For example, if winsor in the eng.sun.com domain sends mail to guy at auspex using smartuucp, the headers look like this:

To: guy@auspex.com
From: winsor@Eng.Sun.COM

Use smartuucp 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 to receive replies from the Internet.

•  uucp uses uux to deliver mail but uses route-based addressing in the headers, in which part or all of the address route is specified by the sender. See "Route-Based Addressing" later in this chapter for more information. For example, if winsor in domain sun.eng.sun.com sends mail to guy@auspex using the uucp mailer, the headers look like this:

To: auspex!guy
From: sun!winsor

The exclamation point (bang) in the address means that it is route-based. Use uucp for uucp connections to systems that need a bang-style path.

•  ddn uses SMTP (Simple Mail Transport Protocol) on TCP port 25 to connect to the remote host; ddn inverts aliases and adds a domain name. For example, if winsor in domain eng.sun.com sends mail to paul@phoenix.princeton.edu, the headers look like this:

To: paul@phoenix.princeton.edu
From: Janice.Winsor@Eng.Sun.COM

If winsor sends mail to irving@sluggo (both are users in the eng.sun.com domain) and is using the main.cf configuration file, the header looks like this:

To: Irving.Who@Eng.Sun.Com
From: Janice.Winsor@Eng.Sun.COM

Use ddn for sending mail outside of your domain, especially for mailers that you must reach through a relay.

•  ether uses the SMTP protocol on port 25 to connect to the remote host; ether does not invert aliases or append a domain name. Use ether for systems in your dns domain that users can reach directly.

You can define other mailers by providing a mailer specification in the sendmail.cf file. See Chapter 4, "Customizing sendmail Configuration Files," for more information.

Domains

A domain is a directory structure for electronic mail addressing and network address naming. The domain address has this format:

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

The part of the address to the left of the @ sign is the local address, as shown in Figure 1-5. The local address may contain information about routing using another mail transport (for example, bob::vmsvax@gateway or smallberries%mill.uucp@physics.uchicago.edu), an alias (iggy.ignatz), or a token that resolves the name of a mailbox (ignatz-->/var/mail/ignatz). The receiving mailer is responsible for determining what the local part of the address means.

Figure 1-5  Domain address structure.

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 upper, lower, or mixed case in the domain part of an address.

The order of domain information is hierarchical, with the locations more specific and local the closer they are to the @ sign (although certain British and New Zealand networks reverse the order).

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NOTE:  Most gateways automatically translate the reverse order of British and New Zealand domain names into the commonly used order. The larger the number of subdomains, the more detailed the information that is provided about the destination. Just as a subdirectory or a file in a file system hierarchy is inside of the directory above, each subdomain is considered to be inside of the one located to its right.

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Table 1-2 shows the top-level domains in the United States.

Because of the increasing popularity of the World Wide Web, the International Ad Hoc Committee (IAHC), a coalition of participants from the broad Internet community, has implemented a proposal to add seven new generic top-level domains (gTLDs) to the existing set. The new gTLDs are listed in Table 1-3.