Knowledgebase
SMTP Response Codes (Reply Codes)
Posted by Sean Syed on 18 February 2013 07:40 PM

Overview
When any mail client or remote server attempts to send email to a mail server, the mail server will send a response to each command. The response indicates whether the command was processed correctly or not. 


This list is a copy of the Theory of Reply Codes as discussed in RFC821 (the Internet Mail Standard). 


 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.


Positive Completion Reply (2.x.x):
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.]

 

  200 (nonstandard success response, see rfc876)
  211 System status, or system help reply
  214 Help message
  220 <domain> Service ready
  221  <domain> Service closing transmission channel
  250 Requested mail action okay, completed  
  251 User not local; will forward to <forward-path>
  283 The Gordano server is configured so that messages aren't being saved. The FROM and TO address are taken from the e-mail message headers



Positive Intermediate (3.x.x):

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. 

  354 Start message input and end with <CRLF>.<CRLF>. This indicates that the server is ready to accept the message itself (after you have told it who it is from and where you want to to go).



Transient Negative Completion Reply(4.x.x):

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.)

  421 <domain> Service not available, closing transmission channel
  450 Requested action not taken; mailbox unavilable or busy  
  451 Requested action aborted, local error in processing
  452  Requested action not taken, insufficient system storage  



Permanent Negative Completion Reply (5.x.x):

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).

  500 Syntax error, command unrecognized. Also command line too long
  501 Syntax error in paramaters or arguements
  502 Command not implemented
  503 Bad sequence of commands
  504 Command parameters not implemented
  521 <domain> does not accept mail (see rfc1846)
  530 Access denied
  550 Action not taken. Mailbox unavailable. Not found, not accessible
  551 User not local, please try
  552 Exceeded storage allocation
  553 Mailbox name not allowed. Mailbox syntax may be incorrect
  554 Transaction failed
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