Message delivery errors
A delivery error is an email returned to the sender because it could not be delivered for some reason. Delivery errors are divided into 2 categories: permanent (hard) and temporary (soft).
The text description of the same error may differ depending on the mail service that returns it. At the same time, it should be remembered that not always the error description really corresponds to the actual problem, and quite often you can observe general messages without specific reasons.
enKod categorizes errors on the basis of the message, not the code, because often the code can say, for example, about a hard error and the message about a soft error.
Hard errors
Hard error codes are usually the 5xx series (Permanent Negative Completion Reply). These codes report that the message did not reach the recipient due to the persistent problems.
Here are some standard hard email error codes and their descriptions:
| Error Code | Error Message | Error Description |
|---|---|---|
| 550 | Non-existent email address or domain | Recipient's email address or domain does not exist |
| 551 | User not local; please try forwarding | The sender is not allowed to send messages to the specified mailbox or the message is being sent to a server that does not accept mail. |
| 552 | Mailbox full; exceeded storage allocation | Memory limits exceeded (too many recipients or too long a message) |
| 553 | Invalid recipient address format | Email address in error or wrong format |
| 554 | Transaction failed; message refused | Generic error code allowing multiple error variants |
| 555 | Syntax error in parameters or arguments | Incorrect syntax in sender or recipient address |
| 556 | Domain does not exist (DNS) | There may be a problem with the sender's DNS. |
| 557 | Your domain does not have a valid MX DNS record | There may be a problem with the sender's DNS. |
| 558 | Mail server requires authentication | Probable problem with sender's DNS |
| lookup error | Achieved max sending try number | Probable problem with sender's DNS |
Probable errors indicating that the recipient's email or domain does not exist:
- Mailbox does not exist. We do not relay
- No such user!
- Unknown user
- Recipient address rejected: Inactive
- Recipient address rejected: User unknown in local recipient table
- The email account that you tried to reach does not exist
- This domain is not in use and does not accept mail
- Sorry, no mailbox here by that name
- No mailbox by that name is currently available
- Requested action not taken: mailbox unavailable
- Message was not accepted - invalid mailbox
- Bad recipient address syntax
- Bad address mailbox syntax
- Bad destination mailbox address: invalid mailbox
- No correct recipients
- We do not relay without RFC2554 authentication
Blocked
This column contains errors related to the sender being blacklisted or failing to pass spam filters. For example:
| Error Code | Error Message | Error Description |
|---|---|---|
| 421 4.7.0 | Messages from IP temporarily deferred due to unexpected volume or user complaints | The IP address is blocked by the recipient's server (Yahoo). The filter is temporary, usually for 4 hours. If the error persists after 48 hours, contact Yahoo. |
| 451 | Blocked - see https://www.spamcop.net/ Achieved max sending try number | The 'blocking' action is based on the recipient's configuration of their server, typically to use the SpamCopDNSBL in a blocking fashion, which SpamCop.net does not recommend. |
| 550 5.7.1 | This message is blocked due to security reason | The recipient's server is blocking your email. There may be problems with DNS, blacklisting, etc. |
| 550 5.7.1 | Service unavailable, Client host blocked using Spamhaus. | Your IP has been blacklisted |
| 553 | Your ip is listed on RBL | Your IP has been blacklisted as an open mail server |
| 554 5.7.1 | This email from has been rejected. The email message was detected as SPAM. | Your IP has been blacklisted |
| 554 5.7.1 | Email cannot be delivered. Reason: Email detected as Spam by spam filters. | The email message has been marked as spam. There may be problems with DNS, domain or IP reputation, or the content of the message itself |
| 554 5.7.1 | Recipient address rejected: Access denied | The email could not be delivered due to authentication issues or security policies of the recipient's server. |
| 554 5.7.1 | Recipient address rejected: Blacklisted | The email could not be delivered due to the security policy of the recipient's server. There may be DNS or reputation problems |
| short response: | spam dmarc dnsbls complaint policy reason policy rejection access denied block blocked blacklisted | Blocking due to incorrect DMARC configuration. DNS problems are likely |
Other possible error texts that may indicate that the sender has been blocked:
- Nemesis ESMTP Service not available No SMTP service IP address is block listed.
- Spam message rejected
- Mail from IP was rejected due to listing in Spamhaus SBL
- Email rejected per SPAM policy
- Mailbox unavailable or access denied
- Unfortunately, messages from IP weren\'t sent. Please contact your Internet service provider since part of their network is on our block list
Soft errors
Soft error codes are usually 4xx series. They indicate that the mail temporarily cannot be delivered to the recipient. Such errors can be corrected. These are: overflowing mailboxes, blocked mailboxes, other errors (non-standard server responses).
Examples of standard email soft error codes and their descriptions:
| Error code | Error message | Error description |
|---|---|---|
| 421 | Service not available, closing transmission channel | Temporary overload of recipient's mail server |
| 422 | The recipient's mailbox is over quota | The recipient's mailbox is out of space |
| 431 | The recipient's server is temporarily unavailable | The recipient's mail server is temporarily unavailable |
| 432 | The recipient's server is not accepting messages at this time | The recipient's mail server is temporarily not accepting messages |
| 450 | Requested action not taken; mailbox unavailable | The recipient's mail server is temporarily unavailable |
| 451 | Temporary server error; try again later | Receiver's mail server temporarily unavailable |
| 452 | • Insufficient system storage • The recipient\'s inbox is out of storage space. | Insufficient space on recipient's mail server |
| 453 | No mail | Temporary problems with recipient's server |
| 454 | Temporary authentication failure | Temporary authentication problem on recipient server |
| 500 | The email account that you tried to reach is inactive. | The recipient is unavailable, has been deactivated or no longer exists |
Other errors
Errors returned to the sender with non-standard or generalized wording for which the specific reason for non-delivery of the message cannot be identified. For example:
| Error Code | Error Message | Error Description |
|---|---|---|
| 421 4.7.0 | too many errors Achieved max sending try number | The maximum number of send attempts has been reached. |
| 500 | Message rejected | Probably not getting through the spam filters |
| 550 | LTD BeGet: user not found. | DNS problems are likely. Configure SPF correctly, check MX records. |
| 550 | relay not permitted | DNS problems are likely. Correctly configure MX records. |
| 550 | Requested action not taken: mailbox unavailable | The email you sent was blocked by the recipient's email hosting server, and returned to you. |
| 550 5.1.1 | Bad destination mailbox address | Check that the recipient's address is correct |
| 553 | We do not relay without RFC2554 authentication. | There is no Simple Mail Transfer Protocol (SMTP) authentication. |
| 554 | Rejected due to the sending MTA\'s poor reputation. | Delivery rejected due to bad reputation of the sender |
| • connection error • send timeout • error: too many errors | Achieved max sending try number | Connection error. The maximum number of attempts to send has been reached. |
| send error | Sending error without a specific reason |