Originator, Delivery, Relay, and Gateway Systems

This specification makes a distinction among four types of SMTP

systems, based on the role those systems play in transmitting

electronic mail. An "originating" system (sometimes called an SMTP

originator) introduces mail into the Internet or, more generally,

into a transport service environment. A "delivery" SMTP system is

one that receives mail from a transport service environment and

passes it to a mail user agent or deposits it in a message store that

a mail user agent is expected to subsequently access. A "relay" SMTP

system (usually referred to just as a "relay") receives mail from an

SMTP client and transmits it, without modification to the message

data other than adding trace information, to another SMTP server for further relaying or for delivery.

A "gateway" SMTP system (usually referred to just as a "gateway")

receives mail from a client system in one transport environment and

transmits it to a server system in another transport environment.

Differences in protocols or message semantics between the transport

environments on either side of a gateway may require that the gateway

system perform transformations to the message that are not permitted

to SMTP relay systems. For the purposes of this specification,

firewalls that rewrite addresses should be considered as gateways,

even if SMTP is used on both sides of them (see RFC 2979 [27]).

Mailbox and Address

As used in this specification, an "address" is a character string

that identifies a user to whom mail will be sent or a location into

which mail will be deposited. The term "mailbox" refers to that

depository. The two terms are typically used interchangeably unless

the distinction between the location in which mail is placed (the

mailbox) and a reference to it (the address) is important. An

address normally consists of user and domain specifications. The

standard mailbox naming convention is defined to be

"local-part@domain"; contemporary usage permits a much broader set of

applications than simple "user names". Consequently, and due to a

long history of problems when intermediate hosts have attempted to

optimize transport by modifying them, the local-part MUST be

interpreted and assigned semantics only by the host specified in the domain part of the address.