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SMTP Reply Codes - Severities and Theory |
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SMTP Reply Code Severities and Theory
The three digits of the reply each have a special significance. The
first digit denotes whether the response is good, bad or incomplete.
An unsophisticated SMTP client, or one that receives an unexpected
code, will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by examining this first digit. An SMTP client
that wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit. The third digit and any supplemental information that may be
present is reserved for the finest gradation of information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action is being
held in abeyance, pending confirmation of the information in this
reply. The SMTP client should send another command specifying
whether to continue or abort the action. Note: unextended SMTP
does not have any commands that allow this type of reply, and so
does not have continue or abort commands.
2yz Positive Completion reply
The requested action has been successfully completed. A new
request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action is being
held in abeyance, pending receipt of further information. The
SMTP client should send another command specifying this
information. This reply is used in command sequence groups (i.e.,
in DATA).
4yz Transient Negative Completion reply
The command was not accepted, and the requested action did not
occur. However, the error condition is temporary and the action
may be requested again. The sender should return to the beginning
of the command sequence (if any). It is difficult to assign a
meaning to "transient" when two different sites (receiver- and
sender-SMTP agents) must agree on the interpretation. Each reply
in this category might have a different time value, but the SMTP
client is encouraged to try again. A rule of thumb to determine
whether a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be successful if repeated
without any change in command form or in properties of the sender
or receiver (that is, the command is repeated identically and the
receiver does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did not
occur. The SMTP client is discouraged from repeating the exact
request (in the same sequence). Even some "permanent" error
conditions can be corrected, so the human user may want to direct
the SMTP client to reinitiate the command sequence by direct
action at some point in the future (e.g., after the spelling has
been changed, or the user has altered the account status).
The second digit encodes responses in specific categories:
x0z Syntax: These replies refer to syntax errors, syntactically
correct commands that do not fit any functional category, and
unimplemented or superfluous commands.
x1z Information: These are replies to requests for information,
such as status or help.
x2z Connections: These are replies referring to the transmission
channel.
x3z Unspecified.
x4z Unspecified.
x5z Mail system: These replies indicate the status of the receiver
mail system vis-a-vis the requested transfer or other mail system
action.
The third digit gives a finer gradation of meaning in each category
specified by the second digit. The list of replies illustrates this.
Each reply text is recommended rather than mandatory, and may even
change according to the command with which it is associated. On the
other hand, the reply codes must strictly follow the specifications
in this section. Receiver implementations should not invent new
codes for slightly different situations from the ones described here,
but rather adapt codes already defined.
For example, a command such as NOOP, whose successful execution does
not offer the SMTP client any new information, will return a 250
reply. The reply is 502 when the command requests an unimplemented
non-site-specific action. A refinement of that is the 504 reply for
a command that is implemented, but that requests an unimplemented
parameter.
The reply text may be longer than a single line; in these cases the
complete text must be marked so the SMTP client knows when it can
stop reading the reply. This requires a special format to indicate a
multiple line reply.
The format for multiline replies requires that every line, except the
last, begin with the reply code, followed immediately by a hyphen,
"-" (also known as minus), followed by text. The last line will
begin with the reply code, followed immediately by <SP>, optionally
some text, and <CRLF>. As noted above, servers SHOULD send the <SP>
if subsequent text is not sent, but clients MUST be prepared for it
to be omitted.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
In many cases the SMTP client then simply needs to search for a line
beginning with the reply code followed by <SP> or <CRLF> and ignore
all preceding lines. In a few cases, there is important data for the
client in the reply "text". The client will be able to identify
these cases from the current context. |
