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End-of-Sale/Renewals for Models 100 and 200

As of October 4th, 2020, new sales for models 100 and 200 of the Barracuda Email Security Gateway ceased. As of November 30, 2023, renewals for models 100 and 200 of the Barracuda Email Security Gateway have ceased. 

SMTP Error Codes

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This is a basic guide to the SMTP return (or 'error') codes you may see for rejected or deferred messages in the Message Log of the Barracuda Email Security Gateway when an outbound message cannot be delivered to or accepted by the destination mail server. Depending on the actual problem it encounters, the destination mail server that did not accept the message may not be able to provide detailed help on how to resolve a problem. In this case the mail server may only provide an SMTP error code in the server's log, or in the text of the message returned to the sender..

To view an SMTP error code for a rejected or deferred message in the Barracuda Email Security Gateway web interface:

  1. Go to the BASIC > Message Log page.
  2. Double click on the message.
  3. In the Delivery Status field you might see Rejected and a Delivery Detail with an SMTP error code. This example shows error 554:


SMTP error codes you'll see in the Delivery Details field of the Message Details popup for a rejected or deferred message include the following, with more details listed in the table below:

  • 420 - The message has been deferred due to suspect content. Try again later.
  • 421 - The service is not available and the connection will be closed.
  • 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. Perhaps contact the the administrator to alert him/her of the issue.
  • 452 - The command has been aborted because the server has insufficient system storage.
  • 454 - TLS encoding is not available due to a temporary condition.
  • 500 - The server could not recognize the command due to a syntax error.
  • 501 - A syntax error was encountered in command arguments.
  • 502 - This command is not implemented.
  • 503 - The server has encountered a bad sequence of commands.
  • 504 - A command parameter is not implemented.
  • 530 - Must issue STARTTLS command.
  • 535 - Authentication failed.
  • 550 - 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 - The recipient is not local to the server. The server then gives a forwarding address to try.
  • 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.


4XX_RVERIFY_DEFER400Deferred: temporary directory error
4XX_SUSPECT_REALTIME420Deferred due to suspect content, please try again later
4XX_TIMEOUT421Error: timeout
4XX_CLIENT_DISCONNECT421Client disconnected
4XX_EINTERNAL421Internal error
4XX_ESEND421Failed to reply to client
4XX_TOO_MANY_ERRORS421Too many errors
4XX_MSG_PER_SESSION421Error: too many messages in one session
4XX_TEMP_FAIL421Service not available, closing transmission channel. Try again later.



Temporary failure. Try again later.

4XX_TEMP_RECIP450Requested action not taken: mailbox unavailable.
4XX_EINTERNAL451Internal error.
4XX_EINTERNAL451Requested action aborted: local error in processing.



Too many recipients.

4XX_TLS_TEMP_FAIL454TLS not available due to temporary reason.
4XX_BLOCKED_RATE454Too many connections from origin (rate control).
4XX_BLOCKED_RATE2454Too many connections to server (rate control).
4XX_SYNTAX_ERROR500Syntax error, command unrecognized.
5XX_INVALID_CHAR_RECIP    500Syntax error - invalid character.

Syntax error in parameters or arguments.

5XX_AUTH_ABORT501Authentication aborted.
5XX_AUTH_MALFORMED501Malformed authentication input.
5XX_AUTH_RESPONSE_BAD501Error: malformed authentication response.
5XX_DATA_LINE_TOO_LONG501Command line too long or no terminating CRLF in line buffer .
5XX_DATA_MISSING_CRLF       501No terminating CRLF in line buffer.

This command is not implemented.

5XX_MISSING_EHLO       503Error: send HELO/EHLO first.
5XX_ALREADY_AUTH503Error: already authenticated.
5XX_AUTH_REQUIRES_TLS503Issue STARTTLS first before using clear text password.

Bad sequence of authentication commands - Try the following: AUTH CRAM-MD5, AUTH LOGIN.


Bad sequence of commands.

5XX_AUTH_REQUIRES_TLS504Error: encryption required for requested authentication mechanism.

Need Fully Qualified Address (FQDN).

5XX_ERROR_PARAM       504Command parameter not implemented.
5XX_AUTH_UNSUPPORTED504Error: unsupported mechanism.
5XX_AUTH_TYPE504Unrecognized authentication type.
5XX_AUTH_REQUIRES_TLS530Authentication required.



Must issue STARTTLS.

5XX_AUTH_FAILED535Authentication failed.
5XX_BLOCKED_RECIP550Recipient email address rejected.
5XX_BLOCKED_IP550Blocked IP address.
5XX_BLOCKED_SENDER2550Sender Rejected.


550Sender email address rejected.

No such domain at this location.

5XX_HANGUP        550Executing hangup request.
5XX_DISCARD       550Executing discard request.



Empty envelope senders not allowed.

5XX_INVALID_SENDER2550Invalid sender address.
5XX_BLOCKED_IP2          550Sender IP address rejected.



Rejecting for Sender Policy Framework (SPF).

5XX_RECIP  550550Requested action not taken: mailbox unavailable.


550Rejecting spoofed message.

Error: message too large.

5XX_MSG_SIZE2552Message size exceeds fixed limit.

Requested action not taken: mailbox name not allowed.


553Requested action not taken.
5XX_INVALID_RECIP      553Requested action not taken: mailbox name not allowed.

Transaction failed.

5XX_BLOCKED_VIRUS554Rejected due to virus.
5XX_BLOCKED_SPAM554Rejected due to spam content.
5XX_BLOCKED_NO_PTR554IP name lookup failed. No PTR record found for the given IP address.
5XX_BLOCKED_RBL554Service unavailable; client host blocked. The final sending domain or IP address may be on a Real Time Blocklist (RBL).


554Rejecting banned file attachment.


554Rejecting password protected file attachment.
5XX_BLOCKED_BBL554Service unavailable; client host [IP or hostname] blocked using Barracuda Reputation;
5XX_BLOCKED_DLP554Rejected due to banned content.
5XX_BLOCKED_SPAM2554Rejected due to banned content.