This article provides a table of reference to help you identify the reasons why an email was bounced. If you find any bounces that might have been classified wrongly then please contact Emarsys Support so that we can analyze it.
About SMTP bounce codes
The information on this page describes the RFC standards, not the actual implementation on MTA/Mail Server side. Mail server administrators can customize bounce code messages.
Notes:
- The traditional SMTP bounce codes are three-digit numbers (e.g., 550). The enhanced bounce codes are three-digits separated by decimal points (e.g., 5.1.1).
- The tables below are not a definitive list of all available bounce codes, only a representation of the most frequent ones.
Enhanced status codes (class sub-codes) (from IANA.org)
| Code | Summary | Description | Reference |
|---|---|---|---|
| 2.XXX.YYY | Success | Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery. | RFC3463 (Standards track) |
| 4.XXX.YYY | Persistent Transient Failure | A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful. In this case, the bounce will be classified as a soft-bounce. | RFC3463 (Standards track) |
| 5.XXX.YYY | Permanent Failure | A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery. | RFC3463 (Standards track) |
For more information on Class Sub-Codes, click here.
5.X Bounce codes (permanent failures)
| Code | Description |
|---|---|
| 5.0.0 | Address does not exist |
| 5.1.0 | Other address status |
| 5.1.1 | Bad destination mailbox address |
| 5.1.2 | Bad destination system address |
| 5.1.3 | Bad destination mailbox address syntax |
| 5.1.4 | Destination mailbox address ambiguous |
| 5.1.5 | Destination mailbox address invalid |
| 5.1.6 | Mailbox has moved |
| 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 |
| 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 |
| 5.4.0 | Other or undefined network or routing status |
| 5.4.1 | No answer from host |
| 5.4.2 | Bad connection |
| 5.4.3 | Routing server failure |
| 5.4.4 | Unable to route |
| 5.4.5 | Network 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 |
| 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 |
For more information, see RFC3463 (file link).
Traditional SMTP status codes
The table below is not a definitive list of all available status codes, only a representation of the most frequent ones.
| Status Code Group |
Group Name |
Description |
|---|---|---|
| 1xx |
Informational |
Request received, continuing process |
| 2xx |
Success |
The action was successfully received, understood, and accepted |
| 3xx |
Redirection |
Further action must be taken in order to complete the request |
| 4xx |
Client Error |
The request contains bad syntax or cannot be fulfilled |
| 5xx |
Server Error |
The server failed to fulfill an apparently valid request |
| Code |
Description |
|---|---|
| 421 |
domain service not available, closing transmission channel |
| 450 |
Requested mail action not taken: mailbox unavailable (e.g., mailbox busy) |
| 451 |
Requested action aborted: error in processing |
| 452 |
Requested action not taken: insufficient system storage |
| 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 |
| 550 |
User’s mailbox was unavailable (“not found”) |
| 551 |
The recipient is not local to the server |
| 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 for some unstated reason |
References:
- For more information on HTML Status Codes, click here.
- For more information, see RFC2821 (File Link).