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. (Notice that this is an example and not an actual route.) If any of the mail handlers is out of commission, 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.