Problem
What is SMTP and understanding the error codes
Solution
Ever wondered why your emails don't send or your email marketing campaign fails and all you're left with is something called an SMTP error code and a short meaningless message?
Before you begin to panic this article breaks down the confusion of the SMTP error messages.
What is SMTP?
First let's discuss what SMTP actually is and how it fits into the world of sending and receiving emails. SMTP stands for Simple Mail Transfer Protocol. SMTP is the mechanism that is used between mail servers to exchange emails. If you would like to know more on the history of SMTP and the first message ever sent click here.
All emails sent by a client, mobile device or server use SMTP to deliver the messages from the source to the destination mail server. Think of SMTP as your local postal service and sorting office ensuring your mail gets to the right location and person.
Once the message has been received by a mail server, the user collects that message using their client application (Outlook, Windows Mail, Mac Mail, etc) or mobile device (iPhone, iPad, Windows Mobile, Android, etc) using a protocol called POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol).
Every time an email is sent using SMTP the receiving server will respond with an SMTP error code. Not all SMTP error codes actually indicate failures, for example an SMTP error code 250 (2.5.0) means the message was accepted and the delivery action was complete.
So not all SMTP error messages are bad. SMTP error codes usually consist of three digits that carry different meanings.
First digit
The first digit has the following meaning
1xx - Command accepted by mail server but confirmation message is required.
2xx - Task was completed without errors.
3xx - Mail server accepted the request but requires more information.
4xx - Temporary failure encountered by mail server and you have to try again. If you see one of these errors, the mail server or client should tell you that it will automatically attempt to resend the email.
5xx - Fatal error encountered, Mail server cannot process request.
Second digit
The second digit gives more information and there are six possible values.
x0x - A syntax error has occurred.
x1x - Indicates an informational reply, for example to a HELP request.
x2x - Refers to the connection status.
x3x - unspecified.
x4x - unspecified.
x5x - Refers to the status of the mail system as a whole and the mail server in particular.
Third digit
The last digit is even more specific and shows more graduations of the mail transfer status.
SMTP error codes
Below is a list of common STMP error codes that you will most likely see when sending emails and using email marketing software such as Mach5 Mailer or SendBlaster to name a few.
2xx codes
211 - A system status message.
214 - A help message for a human reader follows.
220 - SMTP Service ready.
221 - Service closing.
221 - 2.0.0 Goodbye.
235 - 2.7.0 Authentication succeeded.
250 - Requested action taken and completed. This is the best message of them all because it means the message was delivered with no errors.
251 - The recipient is not local to the server, but the server will accept and forward the message.
252 - The recipient cannot be verified (VRFY), but the server accepts the message and attempts delivery.
3xx codes
334 - A Server challenge when the text part of the message contains a Base64-encoded part, See RFC4954 for more information.
354 - Indicates mail server is ready to take message or start mail input and end with a dot.
4xx codes – Temporary Failure: retry will occur
421 - The service is not available and the connection will be closed.
432 - 4.7.12 Password required.
450 - The requested command failed because the user's mailbox was unavailable (for example because it was locked). Try again later.
451 - The command has been aborted due to a server error.
451 - 4.7.1 Received too many messages from a new or untrusted IP Address.
452 - The command has been aborted because the server has insufficient system storage.
454 - Authentication Failure.
454 - The mail server is unable to accommodate parameters.
5xx codes – Fatal Errors: No retry will occur
500 - The server could not recognize the command due to a syntax error.
501 - A syntax error was encountered in command arguments.
501 - 5.5.2 Can not Base64-decode a client repsonse.
502 - This command is not implemented.
503 - The server has encountered a bad sequence of commands.
504 - A command parameter is not implemented.
504 - 5.5.4 Unrecognised authentication type.
510 - Bad Email Address.
521 - Server does not accept mail.
523 - Encryption is required.
530 - 5.7.0 Authentication is required.
534 - 5.7.9 Authentication mechanism is too weak.
535 - 5.7.8 Authentication - invalid credentials.
538 - 5.7.11 Encryption required for requested authentication mechanism.
550 - The requested command failed because the user's mailbox was unavailable. This error can be caused by quite a few situations, some of which are Relay Denied, No such user, Account not available and Host rejected.
551 - The recipient is not local to the server. Also known as a Relay Denied error
552 - The action was aborted due to exceeded storage allocation.
553 - The command was aborted because the mailbox name is invalid.
554 - The transaction failed.
554 - 5.3.4 Message too big.
556 - Domain doesn't accept mail.
More information
For more information on SMTP error codes and the Simple Mail Transfer Protocol as laid down in RFC 821 please visit http://www.faqs.org/rfcs/rfc2821.html
Additional information on POP3 (RFC 1939 & RFC 2449) and IMAP (RFC 3501) can be found below:
