Linux Network Administrator's Guide, Second Edition

Email Addresses

Email addresses are made up of at least two parts. One part is the name of a mail domain that will ultimately translate to either the recipient’s host or some host that accepts mail on behalf of the recipient. The other part is some form of unique user identification that may be the login name of that user, the real name of that user in “Firstname.Lastname” format, or an arbitrary alias that will be translated into a user or list of users. Other mail addressing schemes, like X.400, use a more general set of “attributes” that are used to look up the recipient’s host in an X.500 directory server.

How email addresses are interpreted depends greatly on what type of network you use. We’ll concentrate on how TCP/IP and UUCP networks interpret email addresses.

RFC-822

Internet sites adhere to the RFC-822 standard, which requires the familiar notation of [e-mail], for which host.domain is the host’s fully qualified domain name. The character separating the two is properly called a “commercial at” sign, but it helps if you read it as “at.” This notation does not specify a route to the destination host. Routing of the mail message is left to the mechanisms we’ll describe shortly.

You will see a lot of RFC-822 if you run an Internet connected site. Its use extends not only to mail, but has also spilled over into other services, such as news. We discuss how RFC-822 is used for news in Chapter 20.

Obsolete Mail Formats

In the original UUCP environment, the prevalent ...