About SMTP error messages

SMTP is an internet standard used by mail servers to send and receive email. SMTP error messages help you understand why a message wasn't sent successfully. If incoming or outgoing messages are bouncing, check the bounce messages for an SMTP error code that can help you diagnose the problem.

SMTP error messages follow this format. Each digit in the error and status codes provides an increasing level of detail:

Reply code Status code Reply text
xyz x.y.z Text description

This is an example SMTP error message. The message describes why the error occurred and recommends actions to fix the problem:

SMTP error message examples

Reply codes

The SMTP reply codes refer to specific parts of the email service, and can indicate issues like:

  • The intended recipient isn't registered with the service.
  • The message was marked as spam or as having a virus.
  • The sender’s domain was identified as a source of spam.
  • The message violated your company’s email policy. For example, the message included a credit card number or prohibited language.

Response codes that start with 4 and 5 are errors, and the number indicates the error type. Errors that start with a 4 are temporary failures. No action is needed, the sender will try again. Errors that start with 5 are permanent failures and action is required to fix the problem.

Common SMTP 400 error codes

Error codes that start with a 4 indicate the server had temporary failure but the action will be completed with another try. Error codes that start with 4 are typically errors related to the receiving mail server. 

Error code Description
421 Service isn't available, try again later
450 Requested action wasn't taken because the user's mailbox was unavailable
451 Message not sent because of server error
452 Command stopped because there isn’t enough server storage
455 Server can't deal with the command right now

Common SMTP 500 error codes

Error code Description
500 Server couldn't recognize the command because of a syntax error
501 Syntax error found in command parameters or arguments
502 Command not implemented
503 Server had bad sequence of commands
541 Message rejected by the recipient address
550 Requested command failed because the user’s mailbox was unavailable, or the receiving server rejected the message because it was likely spam
551 Intended recipient mailbox isn't available on the receiving server
552 Message wasn't sent because the recipient mailbox doesn't have enough storage
553 Command stopped because the mailbox name doesn't exist
554 Transaction failed without additional details

SMTP error codes are typically followed by 3 more digits. This is the SMTP enhanced status code, which gives more detail about what caused the failure. The second number in the code indicates the response category. The third character in the code provides the additional details.

Reply code 421 is a temporary error about the transmission channel that contains an informational request.

Reply code Description
x0z Syntax errors
x1z Replies to information requests
x2z Transmission channel errors
x3z Unspecified
x4z Unspecified
x5z Mail system status

Status codes

The first digit in the status code indicates the code class, the second digit indicates the subject, and the third digit provides more detail.

Status code 4.4.5 is a temporary network and routing issue for the mail delivery protocol status.

Status code Description
x.0.z Unidentified
x.1.z Addressing status
x.2.x Mailbox status
x.3.x Mail system status
x.5.x Mail delivery protocol status
x.6.x Message content or media status
x.7.x Security or policy status

Reply text

The last field in an SMTP error message describes the error.

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