ASCII American Standard Code for Information Interchange . command A request for a mail service action sent by the sender-SMTP to the receiver-SMTP. domain The hierarchially structured global character string address of a host computer in the mail system. end of mail data indication A special sequence of characters that indicates the end of the mail data. In particular, the five characters carriage return, line feed, period, carriage return, line feed, in that order. host A computer in the internetwork environment on which mailboxes or SMTP processes reside. line A a sequence of ASCII characters ending with a <CRLF>. mail data A sequence of ASCII characters of arbitrary length, which conforms to the standard set in the Standard for the Format of ARPA Internet Text Messages (RFC 822 ). mailbox A character string (address) which identifies a user to whom mail is to be sent. Mailbox normally consists of the host and user specifications. The standard mailbox naming convention is defined to be "user@domain". Additionally, the "container" in which mail is stored. receiver-SMTP process A process which transfers mail in cooperation with a sender-SMTP process. It waits for a connection to be established via the transport service. It receives SMTP commands from the sender-SMTP, sends replies, and performs the specified operations. reply A reply is an acknowledgment (positive or negative) sent from receiver to sender via the transmission channel in response to a command. The general form of a reply is a completion code (including error codes) followed by a text string. The codes are for use by programs and the text is usually intended for human users. sender-SMTP process A process which transfers mail in cooperation with a receiver-SMTP process. A local language may be used in the user interface command/reply dialogue. The sender-SMTP initiates the transport service connection. It initiates SMTP commands, receives replies, and governs the transfer of mail. session The set of exchanges that occur while the transmission channel is open. transaction The set of exchanges required for one message to be transmitted for one or more recipients. transmission channel A full-duplex communication path between a sender-SMTP and a receiver-SMTP for the exchange of commands, replies, and mail text. transport service Any reliable stream-oriented data communication services. For example, NCP, TCP, NITS. user A human being (or a process on behalf of a human being) wishing to obtain mail transfer service. In addition, a recipient of computer mail. word A sequence of printing characters. <CRLF> The characters carriage return and line feed (in that order). <SP> The space character.