The TCP/IP Guide - SMTP Replies and Reply Codes

Table 255 contains a list of some of the more common SMTP reply codes taken from RFC 2821, in numerical order. For each, I have shown the “typical” reply text specified in the standard, and provided additional descriptive information where I felt it was needed.

**Table 255: SMTP Reply Codes**
Reply Code	Reply Text	Description
211	System status or system help reply.
214	<Help message…>	Used for text sent in reply to the HELP command.
220	<servername> Service ready.	Greeting message sent when TCP connection is first established to an SMTP server.
221	<servername> closing transmission channel.	Goodbye message sent in response to a QUIT message.
250	Requested mail action okay, completed	Indicates successful execution of a variety of commands.
251	User not local; will forward to <forward-path>	Used when the SMTP receiver agrees to forward a message to a non-local user.
252	Cannot VRFY user, but will accept message and attempt delivery	Indicates that a server tried to verify an e-mail address, but was not able to do so completely. Usually means the address appears to be valid but it was not possible to positively ascertain this to be true.
354	Start mail input; end with <CRLF>.<CRLF>	Intermediate reply to a DATA command.
421	<servername> Service not available, closing transmission channel	Sent in response to any command when the SMTP receiver prematurely terminates the connection. A common reason for this is receipt of a local shutdown command, due for example to a hardware reboot.
450	Requested mail action not taken: mailbox unavailable	Sent when a mailbox is busy due to another process accessing it.
451	Requested action aborted: local error in processing	Local processing problem on the server.
452	Requested action not taken: insufficient system storage.	Time to clean out the server's hard disk! J
500	Syntax error, command unrecognized	Response to a bad command or one that was too long.
501	Syntax error in parameters or arguments
502	Command not implemented	Command is valid for SMTP in general but not supported by this particular server.
503	Bad sequence of commands	Commands were not sent in the correct order, such as sending the DATA command before the MAIL command.
504	Command parameter not implemented.
550	Requested action not taken: mailbox unavailable	Generic response given due to a problem with a specified mailbox. This includes trying to send mail to an invalid address, refusal to relay to a non-local mailbox and so forth.
551	User not local; please try <forward-path>	Tells the SMTP sender to try a different path; may be used to support mailbox forwarding.
552	Requested mail action aborted: exceeded storage allocation	User's mailbox is full.
553	Requested action not taken: mailbox name not allowed	Specification of an invalid mailbox address.
554	Transaction failed.	General failure of a transaction.

As mentioned before, the actual text string for each reply code is implementation-specific. While the standard specifies “dry” response text such as “Requested action completed” for a 250 message, some servers customize this code, or even give different replies to different 250 messages depending on context. Again, there are examples of this in other topics.

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