Simple Mail Transfer Protocol (SMTP) responses are sent from server to client. Each response in SMTP begins with three digit code and it may be followed by additional textual information. The leading digits in response indicate category of response. Following are different categories of the responses :
1. Positive completion reply 2. Positive Intermediate reply 3. Transient Negative Completion reply 4. Permanent Negative Completion reply
These are explained as follows :
Positive completion reply –
This reply refers to successfully completion of action. After that new request may be initiated.
Positive Intermediate reply –
This reply refers to acceptance of command which we are given, but requested action is being held in state of suspension and also pending receipt of further information.
Transient Negative Completion reply –
This reply refers to that command was not accepted and requested action did not occur. The error condition is temporary in that case and also action may be requested again.
Permanent Negative Completion reply –
This reply refers that command was not accepted and requested action did not occur.
Various SMTP responses are listed in the following table :
|211||System status and system help reply.|
|221||<domain>service closing transmission channel.|
|250||Requested mail action okay, completed.|
|251||User not local; the message will be forwarded to <forward-path>.|
|354||Start mail input; end with <CRLF>.|
|421||<domain>service not available, losing transmission channel.|
|450||Requested mail action not taken, mailbox not available ( e.g. Mailbox busy ).|
|451||Requested action aborted; local error in processing.|
|452||Requested action not taken; insufficient system storage system.|
|500||Syntax error, command unrecognized.|
|501||Syntax error in parameters or arguments.|
|502||Command not implemented.|
|503||Bad sequence of commands.|
|504||Command parameter not implemented.|
|550||The requested action is not taken due to the unavailability of mailbox ( e.g. Mailbox not found, no access ).|
|551||User not local; please try <forward-path>.|
|552||Requested mail action aborted; exceeded storage allocation.|
|553||The requested action is not taken because the mailbox name does not allowed it (e.g., Mailbox syntax incorrect.).|
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