SMTP servers use to respond to commands using a defined set of codes. This page is a small guide to the codes mail servers may produce when you try to send mail along with their meanings. They are part of the SMTP protocol (the set of rules used to send emails).
The Meaning of the Codes
A mail server will reply to every request a client (such as MaxBulk Mailer) makes with a return code. This code consists of three numbers.
The first generally tells whether the server accepted the command and if it could handle it. The five possible values are:
1: The server has accepted the command, but does not yet take action. A confirmation message is required.
2: The server has completed the task successfully.
3: The server has understood the request, but requires further information to complete it.
4: The server has encountered a temporary failure. If the command is repeated without any change, it might be completed.
5: The server has encountered an error.
The second number gives more information. Its six possible values are:
0: A syntax error has occured.
1: Indicates a informational reply, for example to a HELP request.
2: Refers to the connection status.
3 and 4 are unspecified.
5: Refers to the status of the mail system as a whole and the mail server in particular.
The last number is even more specific and shows more graduations of the mail transfer status.
This leads us to the detailed list of ESMTP server response codes, as layed down in RFC 821 and later extensions.
Code
Description
211
System status, or system help reply.
214
Help message (for a human reader follows).
220
SMTP Service ready (or Ready to start TLS).
221
SMTP Service closing transmission channel.
250
OK, queuing for node node started. Requested mail action okay, completed (Requested action taken and completed).
251
OK, no messages waiting for node node. User not local, will forward to forwardpath (The recipient is not local to the server, but the server will accept and forward the message).
252
Cannot VRFY user (e.g., info is not local), but will take message for this user and attempt delivery (The recipient cannot be verified, but the server accepts the message and to attempt delivery).
253
OK, messages pending messages for node node started.
354
Start mail input; end with <CRLF>.<CRLF>. This indicates that the server is ready to accept the message itself (after you have told the server who the message is from and who you want it to be sent to).
355
Octet-offset is the transaction offset.
421
Domain service not available, closing transmission channel (The service is not available and the connection will be closed).
432
A password transition is needed.
450
Requested mail action not taken: mailbox unavailable.
ATRN request refused (The requested command failed because the user's mailbox was unavailable, for example because it was locked).
451
Requested action aborted: local error in processing. Unable to process ATRN request now (The command has been aborted due to a server error. Not your fault. Maybe let the admin know).
452
Requested action not taken: insufficient system storage (The command has been aborted because the server has insufficient system storage, it happens sometimes when sending to a lot of recipients in Bcc mode).
453
You have no mail.
454
TLS not available due to temporary reason.
Encryption required for requested authentication mechanism.
458
Unable to queue messages for node node.
459
Node node not allowed: reason.
500
Command not recognized: command. Syntax error (The server could not recognize the command due to a syntax error).
501
Syntax error, no parameters allowed (A syntax error was encountered in command arguments).
502
Command not implemented (This command is not implemented).
503
Bad sequence of commands (The server has encountered a bad sequence of commands).
504
Command parameter not implemented (A command parameter is not implemented).
505
Authentication required
521
Machine 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 (The requested command failed because the user's mailbox was unavailable, for example because it was not found, or because the command was rejected for policy reasons).
551
User not local; please try forwardpath (The recipient is not local to the server. The server then gives a forward address to try).
552
Requested mail action aborted: exceeded storage allocation (The action was aborted due to exceeded storage allocation).
553
Requested action not taken: mailbox name not allowed (The command was aborted because the mailbox name is invalid).
554
Transaction failed.
571
No external routing allowed, remote sending only allowed with authentication (Illegal relay, missing authentication, RBL/blacklisting)
572
Bounces are blocked due to system abuse by spammers
▸ Blue rows correspond to successfull responses.
▸ Rose rows correspond to delivery errors
▸ Red rows corresppond to server errors.
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The first generally tells whether the server accepted the command and if it could handle it. The five possible values are:
