Recipe 20.19. Table of SMTP Response Codes and SMTP Commands

Table 20-1 is a quick reference for the different SMTP response codes. Complete information about these is in RFC 2821. I recommend studying RFC 2821, as it describes every step of the SMTP transaction in detail. You can also see these codes in action during a telnet session, as in Recipe 20.4.

Table 20-1. Code categories

Code category	Description
2xx	Positive completion reply. The requested action has been successfully completed.
3xx	Positive intermediate reply. Send more information.
4xx	Transient negative completion reply. The command was not accepted; please try again.
5xx	Permanent negative completion reply. The command was not accepted. Go away and don't try again.

Table 20-2 lists the specific codes.

Table 20-2. Specific codes

Code	Description
211	System status or system help reply.
214	A help message for a human reader follows.
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>.
252	Cannot VRFY user, but will accept message and attempt delivery.
354	Start mail input; end with <CRLF>.<CRLF>.
421	<domain> Service not available, closing transmission channel.
450	Requested mail action not taken: mailbox busy or unavailable.
451	Requested action aborted: local error in processing.
452	Requested action not taken: mailbox full.
501	Syntax error, no parameters allowed.
502	Command not implemented.
503	Bad sequence of commands.
504	Command parameter not implemented.
521	<host> does not accept mail.
530	Must issue a STARTTLS command first. Encryption required for requested authentication mechanism.
534	Authentication mechanism is too weak.
538	Encryption required for requested authentication mechanism.
550	Requested action not taken: mailbox unavailable.
551	User not local; please try <forward-path>.
552	Requested mail action aborted: exceeded storage allocation.
553	Requested action not taken: mailbox name not allowed.
554	Transaction failed.

Table 20-3 lists the SMTP commands.

Table 20-3. SMTP commands

Command	Description
HELLO (HELO)	Identifies the sender-SMTP to the receiver-SMTP by hostname.
MAIL (MAIL)	Identifies the sender, and begins the mail transaction.
RECIPIENT (RCPT)	The name of the person to whom the message is addressed.
DATA (DATA)	The message itself, containing the usual elements that you see in a mail client: From, To, Date, CC, Reply-to.
RESET (RSET)	Aborts the current mail transaction. Any stored sender, recipients, and mail data are discarded, and all buffers and state tables are cleared. The receiver must send an OK reply.
VERIFY (VRFY)	Asks the receiver to confirm that the recipient has a mailbox on the system. Spammers like to exploit this, so Postfix provides a way to disable it (Recipe Recipe 21.4).
NOOP (NOOP)	Asks the receiver to send an OK response.
QUIT (QUIT)	Requests that the receiver send an OK reply and then ends the transmission.