Commands and Replies

SMTP commands and, unless altered by a service extension, message

data, are transmitted from the sender to the receiver via the

transmission channel in "lines".

An SMTP reply is an acknowledgment (positive or negative) sent in

"lines" from receiver to sender via the transmission channel in

response to a command. The general form of a reply is a numeric

completion code (indicating failure or success) usually followed by a

text string. The codes are for use by programs and the text is

usually intended for human users. RFC 3463 [25], specifies further

structuring of the reply strings, including the use of supplemental

and more specific completion codes (see also RFC 5248 [26]).

Lines

Lines consist of zero or more data characters terminated by the

sequence ASCII character "CR" (hex value 0D) followed immediately by

ASCII character "LF" (hex value 0A). This termination sequence is

denoted as <CRLF> in this document. Conforming implementations MUST

NOT recognize or generate any other character or character sequence

as a line terminator. Limits MAY be imposed on line lengths by servers (see Section 4).

In addition, the appearance of "bare" "CR" or "LF" characters in text

(i.e., either without the other) has a long history of causing

problems in mail implementations and applications that use the mail

system as a tool. SMTP client implementations MUST NOT transmit

these characters except when they are intended as line terminators

and then MUST, as indicated above, transmit them only as a <CRLF> sequence.

Message Content and Mail Data

The terms "message content" and "mail data" are used interchangeably

in this document to describe the material transmitted after the DATA

command is accepted and before the end of data indication is

transmitted. Message content includes the message header section and

the possibly structured message body. The MIME specification (RFC

2045 [21]) provides the standard mechanisms for structured message bodies.