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DNS and sendmail

Domain Name Service (DNS) is a required part of the mail services infrastructure. sendmail depends on DNS to transmit e-mail outside of internal networks. sendmail looks for DNS MX records to determine which system is used to transmit e-mail to each destination domain. DNS provides host names to the IP address service and serves as a database for mail administration. Networks generally organize their hosts into a hierarchy of administrative domains.

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

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

The part of the address to the left of the @ sign is the local address. The local address may contain information about routing using another mail transport. For example, if you are on the Internet and you want to send e-mail to a person (jim) who lives in a UUCP domain (joebob.uucp) that is hiding behind a domain that has direct Internet connectivity and DNS (starlight.com), you would use the address jim%joebob.uucp.@starlight.com.

Generally, this kind of addressing is no longer needed and is strictly prohibited by sendmail by default because it is used by spammers to send e-mail through other sites (called spam relaying). For example, suppose you want to send e-mail to ralph at idiot.com but you want to go through starlight.com to get there. Both domains are on the Internet and both run DNS, but you can still address the e-mail to ralph%idiot.com@starlight.com. This syntax sends the message(s) to starlight.com, whose sendmail converts the address to ralph@idiot.com. This conversion eats up time on the starlight.com gateway or mail hub.

The part of the address to the right of the @ sign shows the domain address for the local address. A dot (.) separates each part of the domain ad_dress. 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).


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 the directory above, each subdomain is considered to be inside the one located to its right.

Table 3 shows the top-level domains in the United States.

Table 3 Top-Level Domains in the United States




Commercial sites.


Educational sites.


Government installations.


Military installations.


Networking organizations.


Nonprofit organizations.

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 4.

Table 4 New Generic Top-Level Domains




Entities emphasizing cultural and entertainment activities.


Businesses or firms.


Entities providing information services.


Entities that want individual or personal nomenclature.


Entities emphasizing recreation and entertainment activities.


Businesses offering goods to purchase.


Entities emphasizing activities related to the World Wide Web.

In addition to the new gTLDs, up to 28 new registrars will be established to grant registrations for second-level domain names. To guide future registrar developments, under Swiss law a Council of Registrars (CORE) association will be established to create and enforce requirements for registrar operations. The full text of the IAHC report is available at http://www.iahc.org.


The IAHC was dissolved in 1997. Its successor, linked from the IAHC site, deals exclusively with DNS.

Table 5 shows the top-level domains for the United States and European countries.

Table 5 Top-Level Country Domains












West Germany.




















The Netherlands.










United Kingdom.


United States.

The following examples show education, commercial, and government domain addresses.


The following address is for a French domain.


The following address is for a British domain.


Note that some British and New Zealand networks write their mail addresses from top level to lower level, but most gateways automatically translate the address into the commonly used order (that is, lower level to higher).

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