Email Bounce Codes
The reason for a bounced email can be hard to understand because mail server administrators may compose their own messages to explain bounce codes, or provide no message at all.
It's the bounce code that counts.
A recent Possibilities newsletter bounced with code 451, no explanation. After a bit of searching, 2 relevant RFCs were found that explained bounce codes. This blog post is partly for my reference but mostly so its listed in search engines where you can find it for your reference.
There are traditional 3-digit bounce codes (###) and increased-precision 2-dotted bounce codes (#.#.#). Below, you'll see a table of each.
Generally, bounce codes that begin with the digit 4 indicate the message may be valid but a temporary condition has caused delivery abandonment. Sending the same email in the future may be successful.
Bounce codes that begin with the digit 5 indicate a permanent failure unlikely to be resolved by resending the same message.
Traditional Email Bounce Codes
Resource: RFC2821 (April 2001)
| Code | Message that might accompany code |
|---|---|
| 421 | <domain> Service not available: Closing transmission channel. (The service may be down or know that it must shut down.)
|
| 450 | Requested mail action not taken: Mailbox unavailable. (The mailbox may be busy temporarily.) |
| 451 | Requested action aborted: Error in processing. |
| 452 | Requested action not taken: Insufficient system storage. |
| 500 | Syntax error: Command unrecognized. (May include a command-line-too-long syntax error.) |
| 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. (The mailbox might not exist or access may be blocked.) |
| 551 | The recipient is not local to the server. |
| 552 | User not local: Please try <forward-path>. |
| 553 | Requested action not taken: Invalid mailbox name. |
| 554 | Transaction failed. (An unknown or unstated reason.) |
Email Bounce Codes With Increased Precision
Resource: RFC3463 (January 2003)
| Code | Message that might accompany code |
|---|---|
| 5.0.0 | Undefined status. |
| 5.1.0 | Other address status. (Something about the address caused this error.) |
| 5.1.1 | Bad destination mailbox address. (The specified mailbox, the part to the left of the "@" character in the email address, does not exist.) |
| 5.1.2 | Bad destination system address. (The specified destination, the part to the right of the "@" character in the email address, does not exist or is incapable of accepting mail.) |
| 5.1.3 | Bad destination mailbox address syntax. |
| 5.1.4 | Destination mailbox address ambiguous. |
| 5.1.5 | Destination mailbox address valid. (Not used for bounces, but for positive delivery reports. Included here for linear-numbering completeness.) |
| 5.1.6 | Destination mailbox has moved. No forwarding address. |
| 5.1.7 | Bad sender's mailbox address syntax. |
| 5.1.8 | Bad sender's system address. |
| 5.2.0 | Other or undefined mailbox status. |
| 5.2.1 | Mailbox disabled: Not accepting messages. (This would be a temporary error if the mailbox is re-enabled in the future. But that probability can't be determined from this email bounce code.) |
| 5.2.2 | Mailbox full. |
| 5.2.3 | Message length exceeds administrative limit. |
| 5.2.4 | Mailing list expansion problem/ |
| 5.3.0 | Other or undefined mail system status. |
| 5.3.1 | Mail system full. |
| 5.3.2 | System not accepting network messages. |
| 5.3.3 | System not capable of selected features. |
| 5.3.4 | Message too big for system. |
| 5.3.5 | System incorrectly configured. |
| 5.4.0 | Other or undefined network or routing status. |
| 5.4.1 | No answer from host. |
| 5.4.2 | Bad connection. |
| 5.4.3 | Directory server failure. |
| 5.4.4 | Unable to route. |
| 5.4.5 | Mail system congestion. |
| 5.4.6 | Routing loop detected. |
| 5.4.7 | Delivery time expired. |
| 5.5.0 | Other or undefined protocol status. |
| 5.5.1 | Invalid command. |
| 5.5.2 | Syntax error. |
| 5.5.3 | Too many recipients. |
| 5.5.4 | Invalid command arguments. |
| 5.5.5 | Wrong protocol version. |
| 5.6.0 | Other or undefined media error. |
| 5.6.1 | Media not supported. |
| 5.6.2 | Conversion required and prohibited. |
| 5.6.3 | Conversion required but not supported. |
| 5.6.4 | Conversion with loss performed. (A warning only. Delivery was successful but required a conversion in which some data was lost.) |
| 5.6.5 | Conversion failed. |
| 5.7.0 | Other or undefined security status. |
| 5.7.1 | Delivery not authorized: Message refused. |
| 5.7.2 | Mailing list expansion prohibited. |
| 5.7.3 | Security conversion required but not possible. |
| 5.7.4 | Security features not supported. |
| 5.7.5 | Cryptographic failure. |
| 5.7.6 | Cryptographic algorithm not supported. |
| 5.7.7 | Message integrity failure. |
Those are the status codes as interpreted from the linked RFCs.
Share this post with others who can use the information. And bookmark it for yourself.
Will Bontrager
