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, such as 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 networks interpret email addresses.
Internet sites adhere to the RFC 822 standard, which requires the familiar notation of
Before moving on, let's have a look at the way things used to be. In the original UUCP environment, the prevalent form was path!host!user, for which path described a sequence of hosts the message had to travel through before reaching the destination host. This construct is called the bang path notation because an exclamation mark is colloquially called a "bang."
Other networks had still different means of addressing. DECnet-based networks, for example, used two colons as an address separator, yielding an address of host::user. The X.400 standard uses an entirely different scheme, describing a recipient by a set of attribute-value pairs, such as country and organization.
Lastly, on FidoNet, each user was identified by a code such as 2:320/204.9, consisting of four numbers denoting zone (2 for Europe), net (320 referred to Paris and Banlieue), node (the local hub), and point (the individual user's PC). Fidonet addresses were mapped to RFC 822; the above, for example, was written as