💵 Enter the coupon code BLACKFRIDAY before 30th of November for a 25% discount on any new SMS Server license license 💵

How can we help?

List of SMTP response and error codes


The SMTP protocol (Simple Mail Transfer Protocol) is a standard for sending E-mail messages. The SMTP protocol cannot be used for receiving. The IETF (Internet Engineering Task Force) keeps the standard in document RFC 5321.

SMTP is a challenge-response type protocol. Meaning that the client sends a challenge and the server responds. A response always starts with a 3 digit code which contains the general meaning of the response.

The first digit of an SMTP server response determines the response category:

  • 2xx (Success): The action was successful.
  • 3xx (Additional data required): Need more information.
  • 4xx (Transient Error): The action was unsuccessful but it makes sense to try again later.
  • 5xx (Permanent Error): The action was unsuccessful and it does not make sense to try again.

200 – Success codes

Code Description
211 System status, or system help reply
214 Help/Info message; Intended for the human user
220 Service ready
221 Service closing transmission channel
250 Requested mail action completed
251 User not local; message will be forwarded
252 Cannot verify user, but will accept message and attempt delivery

300 – Additional data required

Code Description
354 Start mail input; The server expects the e-mail body

400 – Transient errors

Code Description
421 Service not available, closing transmission channel; When the server is shutting down
450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
451 Requested action aborted: error in processing
452 Requested action not taken: insufficient system storage
455 Server unable to accommodate parameters

500 – Permanent errors

Code Description
500 Syntax error, command unrecognized (This may include errors such as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
551 User not local; The server will not forward
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)
554 Transaction failed (Or, in the case of a connection-opening response, “No SMTP service here”)
555 MAIL FROM/RCPT TO parameters not recognized or not implemented