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 sender-SMTP will be able to determine its next action (proceed as planned, redo, retrench, etc.) by simply examining this first digit. A sender-SMTP that wants to know approximately what kind of error occurred (e.g., mail system error, command syntax error) may examine the second digit, reserving the third digit for the finest gradation of information.
There are five values for the first digit of the reply code:
The command has been accepted, but the requested action is being held in abeyance, pending confirmation of the information in this reply. The sender-SMTP should send another command specifying whether to continue or abort the action.
[Note: SMTP does not have any commands that allow this type of reply, and so does not have the continue or abort commands.]
The requested action has been successfully completed. A new request may be initiated.
The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information. The sender-SMTP should send another command specifying this information. This reply is used in command sequence groups.
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- SMTPs) must agree on the interpretation. Each reply in this category might have a different time value, but the sender-SMTP is encouraged to try again. A rule of thumb to determine if a reply fits into the 4yz or the 5yz category (see below) is that replies are 4yz if they can be repeated without any change in command form or in properties of the sender or receiver. (E.g., the command is repeated identically and the receiver does not put up a new implementation.)
The command was not accepted and the requested action did not occur. The sender-SMTP 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 sender-SMTP 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:
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 sender-SMTP any new information will return a 250 reply. The response 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 sender-SMTP 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>.
For example: 123-First line 123-Second line 123-234 text beginning with numbers 123 The last line
In many cases the sender-SMTP then simply needs to search for the reply code followed by <SP> at the beginning of a line, and ignore all preceding lines. In a few cases, there is important data for the sender in the reply "text". The sender will know these cases from the current context.