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.
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.