System Administration Guide: Network Services

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. ...

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).