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Common Bounce Codes

게시 일자: Aug 5, 2026
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Common Bounce Codes

A bounce code is a reply from a recipient’s mail server that describes why an email wasn’t delivered. Mail server administrators can customize the messages that accompany codes, so bounce messages can be different from server to server. Account Engagement doesn’t generate bounce codes, so contact the recipient’s mail server administrator if you need clarification about a message.

Your bounce reports can show both traditional and enhanced codes. Traditional SMTP bounce codes are three digits. Enhanced SMTP bounce codes are three digits separated by decimal points. Popular services such as Gmail and Outlook.com often have their own bounce responses, so make sure to check the service’s documentation for more. 

Here are some common bounce codes you might see.

Traditional Bounce Codes

CodeExplanation
421<domain> Service not available, closing transmission channel.
450Requested mail action not taken: mailbox unavailable (for example: mailbox busy).
451Requested action aborted: error in processing.
452Requested action not taken: insufficient system storage
500The server couldn’t recognize the command due to a syntax error.
501A syntax error was encountered in command arguments.
502This command isn’t implemented.
503The server has encountered a bad sequence of commands.
504A command parameter isn’t implemented.
550User’s mailbox was unavailable (such as not found).
551The recipient isn’t local to the server.
552The action was aborted due to exceeded storage allocation.
553The command was aborted because the mailbox name is invalid.
554The transaction failed for some unstated reason.

Enhanced Bounce Codes

CodeExplanation
5.0.0Unknown issue
5.1.0Other address status
5.1.1Bad destination mailbox address
5.1.2Bad destination system address
5.1.3Bad destination mailbox address syntax
5.1.4Destination mailbox address ambiguous
5.1.5Destination mailbox address valid
5.1.6Mailbox has moved
5.1.7Bad sender’s mailbox address syntax
5.1.8Bad sender’s system address
5.2.0Other or undefined mailbox status
5.2.1Mailbox disabled, not accepting messages
5.2.2Mailbox full
5.2.3Message length exceeds administrative limit.
5.2.4Mailing list expansion problem
5.3.0Other or undefined mail system status
5.3.1Mail system full
5.3.2System not accepting network messages
5.3.3System not capable of selected features
5.3.4Message too big for system
5.4.0Other or undefined network or routing status
5.4.1No answer from host
5.4.2Bad connection
5.4.3Routing server failure
5.4.4Unable to route
5.4.5Network congestion
5.4.6Routing loop detected
5.4.7Delivery time expired
5.5.0Other or undefined protocol status
5.5.1Invalid command
5.5.2Syntax error
5.5.3Too many recipients
5.5.4Invalid command arguments
5.5.5Wrong protocol version
5.6.0Other or undefined media error
5.6.1Media not supported
5.6.2Conversion required and prohibited
5.6.3Conversion required but not supported
5.6.4Conversion with loss performed
5.6.5Conversion failed
5.7.0Other or undefined security status
5.7.1Delivery not authorized, message refused
5.7.2Mailing list expansion prohibited
5.7.3Security conversion required but not possible
5.7.4Security features not supported
5.7.5Cryptographic failure
5.7.6Cryptographic algorithm not supported
5.7.7Message integrity failure

 

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