SMTP BOUNCE CODES
SMTP Bounce codes are returned by the different mail servers indicating why they didn't accept the emails that you sent it. It can be for many different reasons, having an email bounce is not always an indicator of an email being invalid. It can be for many other reasons, the most common is that the spam filters identified the message as "Spam Content" and rejected it.
In order to determine why the emails that were sent bounced, it's important to have access to these codes, so you can make adjustments and decisions to remedy the issue.
Below you will find a summary of codes, keep in mind, there are thousands more, depending on the mail server the same codes will have different meanings, these are just a few examples.
Reference: RFC2821
5.X BOUNCE CODES (PERMANENT FAILURES)
- CodeDescription
- 5.0.0Address does not exist
- 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 - the address is invalid
- 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
4.X BOUNCE CODES (TEMPORARY FAILURES)
- CodeDescription
- 421service not available, closing transmission channel
- 450Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
- 451Requested action aborted: error in processing
- 452Requested action not taken: insufficient system storage