where a domain is a hierarchical dot-separated list of subdomains. These examples are valid forms of the same address:
eric@monet.berkeley.edu
Eric Allman <eric@monet.berkeley.edu>
eric@monet.berkeley.edu (Eric Allman)
The domain part ("monet.berkeley.edu") is a mail-accepting domain. It can be a host and in the past it usually was, but it doesn't have to be. The domain part is not case sensitive.
The local part ("eric") is often a username, but its meaning is defined by the local software. Sometimes it is case sensitive, although that is unusual. If you see a local-part that looks like garbage, it is usually because of a gateway between an internal e-mail system and the net, here are some examples:
"surname/admd=telemail/c=us/o=hp/prmd=hp"@some.where
USER%SOMETHING@some.where
machine!machine!name@some.where
I2461572@some.where
(These are, respectively, an X.400 gateway, a gateway to an arbitrary internal mail system that lacks proper internet support, an UUCP gateway, and the last one is just boring username policy.)
The real-name part ("Eric Allman") can either be placed before <>, or in () at the end. (Strictly speaking the two aren't the same, but the difference is beyond the scope of this page.) The name may have to be quoted using "", for example, if it contains ".":
"Eric P. Allman" <eric@monet.berkeley.edu>
Many mail systems let users abbreviate the domain name. For instance, users at berkeley.edu may get away with "eric@monet" to send mail to Eric Allman. This behavior is deprecated. Sometimes it works, but you should not depend on it.
In the past, sometimes one had to route a message through several hosts to get it to its final destination. Addresses which show these relays are termed "route-addrs". These use the syntax:
<@hosta,@hostb:user@hostc>
This specifies that the message should be sent to hosta, from there to hostb, and finally to hostc. Many hosts disregard route-addrs and send directly to hostc.
Route-addrs are very unusual now. They occur sometimes in old mail archives. It is generally possible to ignore all but the "user@hostc" part of the address to determine the actual address.
Every site is required to have a user or user alias designated "postmaster" to which problems with the mail system may be addressed. The "postmaster" address is not case sensitive.
RFC 2822 (Internet Message Format)