- If email tracking is turned on in FLG, you may occasionally see an email marked as Failed.
- A Failed email means one of the following happened:
- We couldn't connect to the recipient's mail server
- The mail server rejected our message (sometimes after multiple retries)
- The email was Dropped
- A Dropped email happens when a previous attempt to the same address failed with a permanent error.
- To see more detail, open the email and check the Status field - it will show either a status code or Dropped.
Types of SMTP Status Codes
- Basic codes - a 3-digit code (e.g. 550)
- Enhanced codes - 3 digits separated by decimal points (e.g. 5.1.7)
- You may see either or both formats, depending on the information the receiving server returns.
Basic SMTP Server Status Codes
- Defined in RFC 5321.
| Code | Meaning |
|---|---|
| 421 | Service unavailable - try again later |
| 450 | Action not taken - recipient's mailbox unavailable |
| 451 | Message not sent due to a server error |
| 452 | Command aborted - insufficient server storage |
| 455 | Server cannot process the command at this time |
| 500 | Command not recognised - syntax error |
| 501 | Syntax error in command parameters or arguments |
| 502 | Command not implemented |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
| 550 | Recipient's mailbox address is invalid |
| 551 | Recipient is not local to the server |
| 552 | Action aborted - storage allocation exceeded |
| 553 | Command aborted - mailbox name invalid |
| 554 | Transaction failed for an unstated reason |
| 555 | MAIL FROM / RCPT TO parameters not recognised or not implemented |
Enhanced SMTP Server Status Codes
- Defined in RFC 3463, using the format <class>.<subject>.<detail>.
- Class:
- 2 - Success
- 4 - Temporary delivery error
- 5 - Permanent delivery error
- Subject:
- 1 - Addressing
- 2 - Mailbox (recipient)
- 3 - Mail system (destination)
- 4 - Network and routing
- 5 - Mail delivery protocol
- 7 - Security or policy
- Detail - a 1 to 3 digit number that further classifies the error.
Common Enhanced Status Codes
- The table below covers the most frequent delivery failure codes. X represents the class digit (2, 4, or 5).
| Code | Description |
|---|---|
| X.0.0 | Other undefined status |
| X.1.0 | Other address status |
| X.1.1 | Bad destination mailbox address - it doesn't exist |
| X.1.2 | Bad destination system address |
| X.1.3 | Bad destination mailbox address syntax |
| X.1.4 | Destination mailbox address is ambiguous |
| X.1.6 | Mailbox has moved - no forwarding address |
| X.1.7 | Bad sender's mailbox address syntax |
| X.1.8 | Bad sender's system address |
| X.1.9 | Message relayed to a non-compliant mailer |
| X.1.10 | Recipient address has a null MX |
| X.2.0 | Other or undefined mailbox status |
| X.2.1 | Mailbox disabled - not accepting messages |
| X.2.2 | Mailbox full |
| X.2.3 | Message length exceeds administrative limit |
| X.2.4 | Mailing list expansion problem |
| X.3.0 | Other or undefined mail system status |
| X.3.1 | Mail system full |
| X.3.2 | System not accepting network messages |
| X.3.3 | System not capable of the selected features |
| X.3.4 | Message too big for the system |
| X.3.5 | System incorrectly configured |
| X.4.0 | Other or undefined network or routing status |
| X.4.1 | No answer from host |
| X.4.2 | Bad connection |
| X.4.3 | Directory server failure |
| X.4.4 | Unable to route the message |
| X.4.5 | Mail system congestion |
| X.4.6 | Routing loop detected |
| X.4.7 | Delivery time expired |
| X.5.0 | Other or undefined protocol status |
| X.5.1 | Invalid command |
| X.5.2 | Syntax error |
| X.5.3 | Too many recipients specified |
| X.5.4 | Invalid command arguments |
| X.5.5 | Wrong protocol version |
| X.5.6 | Authentication exchange line too long |
| X.6.0 | Other or undefined media error |
| X.6.1 | Media not supported |
| X.6.2 | Conversion required and prohibited |
| X.6.3 | Conversion required but not supported |
| X.6.5 | Conversion failed |
| X.6.6 | Message content not available |
| X.6.7 | Non-ASCII addresses not permitted |
| X.6.8 | UTF-8 string reply required but not permitted |
| X.6.9 | UTF-8 header message cannot be transferred |
| X.7.0 | Other or undefined security status |
| X.7.1 | Delivery not authorised - message refused |
| X.7.2 | Mailing list expansion prohibited |
| X.7.3 | Security conversion required but not possible |
| X.7.4 | Security features not supported |
| X.7.5 | Cryptographic failure |
| X.7.6 | Cryptographic algorithm not supported |
| X.7.7 | Message integrity failure |
| X.7.8 | Authentication credentials invalid |
| X.7.9 | Authentication mechanism too weak |
| X.7.10 | Encryption needed |
| X.7.12 | Password transition needed |
| X.7.13 | User account disabled |
| X.7.14 | Trust relationship required |
| X.7.15 | Priority level too low |
| X.7.16 | Message too big for the specified priority |
| X.7.17 | Mailbox owner has changed |
| X.7.18 | Domain owner has changed |
| X.7.19 | RRVS test cannot be completed |
| X.7.20 | No passing DKIM signature found |
| X.7.21 | No acceptable DKIM signature found |
| X.7.22 | No valid author-matched DKIM signature found |
| X.7.23 | SPF validation failed |
| X.7.24 | SPF validation error |
| X.7.25 | Reverse DNS validation failed |
| X.7.26 | Multiple authentication checks failed |
| X.7.27 | Sender address has a null MX |
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