Email Bounces
Emails bounces – when an email can’t be delivered for one reason or another – almost like throwing a ball to a someone, only to hit a wall and the ball bounces back at you.
There are two different types of bounces when it comes to email marketing: –
- A Soft Bounce – temporary error
- A Hard Bounce – permanent error
A Soft bounce happens when the person who your are emailing has simply run out of space in their mailbox, the address exists but it simply can’t fit in another email until they clean it up. If this continues, then the system will change to a Hard Bounce as it begins to affect your sending reputation if these emails keep getting bounced back.
Hard Bounce means that there was a fundamental issue delivering your email which one of the following categories: –
Bounce Category: bad-domain
Explanation: The message bounced due to an invalid or non-existing domain, 5.X.X error.
Resolution: Check the spelling and format of the domain, and edit the
contact’s details with the corrected domain. Once the contact has been
updated, their bounce status will be reset and you can send to them
again.
Bounce Category: bad-mailbox
Explanation: The message bounced due to a bad, invalid, or non-existent recipient address, 5.X.X error.
Resolution: Check the spelling and format of the contact’s email
address, and edit with the corrected information. Once the contact has
been updated, their bounce status will be reset and you can send to them
again.
Bounce Category: inactive-mailbox
Explanation: The message bounced due to an expired, inactive, or disabled recipient address, 5.X.X error.
Resolution: Confirm with the contact if their email address is still
active. Check the spelling and format of the contact’s email address,
and edit with the corrected information. Once the contact has been
updated, their bounce status will be reset and you can send to them
again.
Bounce Category: message-expired
Explanation: The message bounced due to not being delivered before the
two day window in which the system continues to attempt delivery, 4.X.X
error.
Resolution: Confirm with the contact if their email address is still
valid, and edit the contact’s email address with any updated
information. If the email address has not changed, ask the contact to
whitelist. Once that has been done, reset the contact’s delivery status
and you will be able to send to them again.
Bounce Category: no-answer-from-host
Explanation: The message bounced due to receiving no response from the
recipient’s server after connecting, 4.X.X or 5.X.X error.
Resolution: The recipient’s mail server is offline or has an issue in
its configuration. Ask the recipient to investigate if there is an issue
with their mail server. Once that has been done, reset the contact’s
delivery status and you will be able to send to them again.
Bounce Category: other messages
Explanation: The message bounced due to other reasons, 4.X.X or 5.X.X error.
Resolution: The system did not receive any detailed message back from
the recipient’s mail server. Ask the contact to whitelist. Once that has
been done, reset the contact’s delivery status and you will be able to
send to them again.
Bounce Category: policy-related
Explanation: The message bounced due to a policy reason on a recipient address or domain, 5.X.X error.
Resolution: This is usually due to a message being classified as spam
by a recipient’s mail server or due to some type of limit restriction in
place on the recipient’s side. Ask the contact to whitelist. Once that
has been done, reset the contact’s delivery status and you will be able
to send to them again.
Bounce Category: quota-issues
Explanation: The message bounced due to the recipient’s mailbox being over its limit, 4.X.X or 5.X.X error.
Resolution: Confirm with the contact if they are able to receive email.
Once that issue has been resolved on their side, reset the contact’s
delivery status and you will be able to send to them again.
Bounce Category: routing-errors
Explanation: The message bounced due to mail routing issues for a recipient domain, 5.X.X error.
Resolution: This is usually due to an invalid configuration in the DNS
records on the recipient’s mail server. Ask the recipient to investigate
if there is an issue with their mail server. Once that has been done,
reset the contact’s delivery status and you will be able to send to them
again.
Bounce Category: spam-related
Explanation: The message bounced due to spam related reasons, 5.X.X error.
Resolution: This is usually due to a message being classified as spam by a recipient’s mail server. Ask the contact to whitelist. Once that has been done, reset the contact’s delivery status and you will be able to send to them again.
Error overview
| Error Code | Error Message | Recommended actions |
| bad-configuration | Messages were rejected due to problems in the remote server configuration. This is a problem on the recipient’s server that has an error in the settings and is not allowing new messages. | You will need to contact the subscriber by phone to talk to the system administrator to try to solve the problem. When the administrator notify that the issue has been solved, you should contact the Mailrelay technical staff. |
| bad-connection | Messages were rejected due to connection problems with the remote server. It is not possible to connect with the recipient’s domain correctly due to some failure in the connection route. In this case, the problem may not be occurring directly on the recipient’s server, but on some intermediate server that is receiving and forwarding the message. | You sould contact the administrator of the recipient’s server to explain the problem, but also with the internet service provider, so they can analyze and try to locate the problem that is causing this error. |
| bad-domain | Messages bounced because the domain is invalid or nonexistent. If the recipient’s domain does not exist, you can receive this error. You need to confirm that the account is valid. | You also will need to contact the subscriber by phone to talk to the system administrator to try to solve the problem. When the administrator notify that the issue has been solved, you should contact the Mailrelay technical staff to validate the email again. |
| bad-mailbox | Messages rejected because the recipient’s address is incorrect, is invalid or nonexistent. The email account was not found on the recipient’s server. | You will need to contact the subscriber by phone to talk to the system administrator to try to solve the problem. When the administrator notify that the issue has been solved, you should contact the Mailrelay technical staff to validate the email again. |
| content-related | Messages bounced due to its content (Possible spam, sender is blacklisted, etc) | You will need to contact the administrator of the recipient’s server for information about the problem and determine possible solutions. And, you should contact our Customer Service Department to see what could be done to solve this issue. |
| inactive-mailbox | Messages rejected because the accounts expired, are inactive or have been disabled. | You will need to confirm that the account is correct and contact the subscriber by phone to talk to the system administrator to try to solve the problem. When the administrator notify that the issue has been solved, you should contact the Mailrelay technical staff. |
| message-expired | The message was sent before the maximum time allowed, after previous attempts have failed. | You can try to send the message again when there is less saturation in the sending queue. |
| no-answer-from-host | Messages bounced because the system could not receive any response from the remote server after connecting. This could be a problem on the remote server configuration. | You will need to contact the administrator of the server to ask for more information on how to solve the problem. |
| other | Were rejected due to other reasons | You will need to contact the administrator of the server to ask for more information on how to solve the problem. |
| policy-related | Messages bounced or blocked due to problems with the policy of the recipient’s server. | You will also need to contact the administrator of the server to ask for more information on how to solve the problem. And, you should contact our Customer Service Department to see what could be done to solve this issue. |
| protocol-errors | The message is being rejected due to syntax errors in the SMTP protocol. Mailrelay always uses correct syntax and commands, but there may be an interpretation problem on the recipient’s server. | You will need to contact the system administrator to try to solve the problem. |
| quota-issues | Messages rejected or blocked due to quota issues. The server rejects the message because the account has exceeded the limit of messages. | You will need to contact the subscriber by phone to talk to the system administrator to try to solve the problem. When the administrator notify that the issue has been solved, you should contact the Mailrelay technical staff to validate the email again. |
| relaying-issues | Messages rejected or blocked due to problems when trying to deliver the emails on the remote server. These errors may occur due to various reasons of internal administrative policies of the recipient’s server, for example, some servers will not accept any email with your domain that does not come from your own IP. | You will need to contact the system administrator to try to solve the problem. |
| routing-errors | Messages due to routing problems in the recipient’s domain. That is, the sender server cannot deliver the email correctly due to a failure in an intermediate server. | You sould contact the administrator of the recipient’s server to explain the problem, but also with the internet service provider, so they can analyze and try to locate the problem that is causing this error. |
| spam-related | Messages blocked or rejected due to reasons related to SPAM. There is a problem with the configurations of the sender domain, the contents of the email or spam complaints and the spam filter is blocking the email. | You will need to contact the administrator of the recipient’s server for information about the problem and determine possible solutions. |
| virus-related | Messages blocked or rejected due to reasons related to infected emails. This can be a serious issue. When you attach a file containing a virus or that potentially could contain a threat, for example, Word or Excel, your emails will probably be blocked. | The best solution in these cases is to scan the attached files using a good antivirus and if the problem occurs because the server rejects the message, marking it as potentially dangerous, you can compress the file using a software like Winzip, Winrar, and upload it to a server. After that, just add a link were your subscribers can download it. |
Generic Error codes
Invalid or expired email address
421 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
550 User’s mailbox was unavailable (such as not found)
551 The recipient is not local to the server.
553 The command was aborted because the mailbox name is invalid.
5.0.0 Address does not exist
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 valid
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
5.1.0 Other address status
5.3.0 Other or undefined mail system status
5.4.1 No answer from host
5.4.2 Bad connection
5.4.0 Other or undefined network or routing status
5.4.3 Routing server failure
5.4.4 Unable to route
5.4.7 Delivery time expired
5.5.0 Other or undefined protocol status
Mailbox full
5.2.2 Mailbox full
5.3.1 Mail system full
Message rejected
5.2.3 Message length exceeds administrative limit.
5.2.4 Mailing list expansion problem
5.3.4 Message too big for system
5.5.3 Too many recipients
5.7.0 Other or undefined security status
5.7.1 Delivery not authorized, message refused
5.7.2 Mailing list expansion prohibited
5.7.7 Message integrity failure
Miscellaneous issues in the remote mail server
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.
552 The action was aborted due to exceeded storage allocation.
554 The transaction failed for some unstated reason.
5.3.2 System not accepting network messages
5.3.3 System not capable of selected features
5.4.5 Network congestion
5.4.6 Routing loop detected
5.5.1 Invalid command
5.5.2 Syntax error
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
5.7.3 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
Yahoo SMTP codes
| SMTP Error | Diagnostic Error Message | Reason |
|---|---|---|
| 421 | 421 Message temporarily deferred – [numeric code] | The message content contained objectionable content, we’re seeing unusual traffic from your servers, or emails from your mail server are generating complaints from Yahoo Mail users. |
| 421 | 421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints – 4.16.55 | We’re seeing unusual traffic patterns from your server. Submit your sending IPs for review. |
| 421 | 421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred due to user complaints – 4.16.55 | We’re seeing unusual traffic patterns from your server or your mailings are generating complaints from Yahoo Mail users. Submit your sending IPs for review. |
| 421 | 421 4.7.1 [TS03] Messages from x.x.x.x permanently deferred. Retrying will NOT succeed | We’re seeing a high volume of e-mails from your server that are indicative of unsolicited mailings. Submit your sending IPs for review. |
| 451 | 451 Resources temporarily not available – Please try again later [numeric code] | This error indicates that our servers were busy and temporarily unable to process your transaction at the time of connection. |
| 451 | 451 VS1-IP Excessive unknown recipients – possible Open Relay (#4.1.8) | Your mail server is configured as an open relay or proxy. Review our guidelines for avoiding deprioritization. |
| 451 | 451 VS1-MF Excessive unknown recipients – possible Open Relay (#4.4.5) | The senders MAIL FROM address has been sending to excessive bouncing accounts (non-active, don’t exist). Review your mailing lists, and remove any addresses that generate bounces. |
| 553 | 553 5.7.1 [BLXX] Connections not accepted from IP addresses on Spamhaus PBL | The sending IP is listed on a Spamhaus blacklist. You can check your status on Spamhaus’ site. |
| 554 | 554 Message not allowed – [numeric code] | Your emails have become deprioritized due to the message content triggering our filters. We ask that you review the email’s header and message content for potentially objectionable content. |
| 554 | Delivery error: dd This user doesn’t have a yahoo.com account (******@yahoo.com) [-5] – mta1126.mail.gq1.yahoo.com [BODY] | The Yahoo account that you’re trying to send to doesn’t exist. We recommend that you contact the recipient directly to confirm their correct email address. |
| 554 | 554 5.7.9 Message not accepted for policy reasons | Your message wasn’t delivered because Yahoo was unable to verify that it came from a legitimate email sender. |
Outlook Error codes
| SMTP Error Code | Explanation |
| 421 RP-001 | The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 421 RP-002 | The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed on this connection. Reason for rate limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 421 RP-003 | The mail server IP connecting to Outlook.com server has exceeded the connection limit allowed. Reason for limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 SC-001 | Emails rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 SC-002 | Emails rejected by Outlook.com for policy reasons. The email server IP connecting to Outlook.com has exhibited namespace mining behaviour. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 SC-003 | Emails rejected by Outlook.com for policy reasons. Your IP address appears to be an open proxy/relay. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 SC-004 | Emails rejected by Outlook.com for policy reasons. A block has been placed against your IP address because we have received complaints concerning emails coming from that IP address. We recommend enrolling in our Junk Email Reporting Programme (JMRP), a free programme intended to help senders remove unwanted recipients from their email list. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 DY-001 | Emails rejected by Outlook.com for policy reasons. We generally do not accept emails from dynamic IPs as they are not typically used to deliver unauthenticated SMTP email to an Internet mail server. If you are not an email/network admin please contact your Email/Internet Service Provider for help. http://www.spamhaus.org maintains lists of dynamic and residential IP addresses. |
| 550 DY-002 | Emails rejected by Outlook.com for policy reasons. The likely cause is a compromised or virus-infected server/personal computer. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
| 550 OU-001 | Emails rejected by Outlook.com for policy reasons. If you are not an email/network admin please contact your Email/Internet Service Provider for help. For more information about this block and to request removal please go to: http://www.spamhaus.org. |
| 550 OU-002 | Emails rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help. |
NOTE: If you are having problems with email delivery to Cox, and you were unable to resolve your problem using the steps in the Error Codes section above, send us an email at unblock.request@cox.net for assistance. Your email must contain all of the following information in order for us to process your request.
COX Email error codes
| Error Code | Description | Resolution |
|---|---|---|
| CXBL | The sending IP address has been blocked by Cox due to exhibiting spam-like behavior. | Send an email request to Cox to ask for a sending IP address be unblocked. Note: Cox has sole discretion whether to unblock the sending IP address. |
| CXTHRT | Email sending limited due to suspicious account activity. To secure your account, please reset your password at cox.com/password. It may take up to 2 hours to confirm security and remove sending limits. | Reset your password at www.cox.com/password or by logging into your Cox.com account and going through the My Profile page. It can take up to 2 hours for the new password to be effective and the email sending limits to be taken off. |
| CXMJ | Email sending blocked due to suspicious account activity on primary Cox account. To secure your account, please reset all Cox account passwords at cox.com/password. Contact Cox at cox.com/chat to remove block and reference error code AUP#CXMJ. | Reset your password at www.cox.com/password or visit www.cox.com to sign in to your primary Cox account and go to the My Profile page. Then, contact Cox at www.cox.com/chat to remove the block referenced by error code AUP#CXMJ. |
| CXDNS | There was an issue with the connecting IP address Domain Name System (DNS). | The Reverse DNS (rDNS) lookup for your IP address is failing. Confirm the IP address that sends your email. Check the rDNS of that IP address. If it passes, then wait 24 hours and try resending your email. |
| CXSNDR | There was a problem with the sender’s domain. | Your email failed authentication checks against your sending domain’s SPF, DomainKeys, or DKIM policy. |
| CXSMTP | There was a violation of SMTP protocol. | Your email wasn’t delivered because Cox was unable to verify that it came from a legitimate email sender. |
| CXCNCT | There was a connection issue from the IP address. | There is a limit to the number of concurrent SMTP connections per IP address to protect the systems against attack. Ensure that the sending email server is not opening more than 10 concurrent connections to avoid reaching this limit. |
| CXMXRT | The sender has sent email to too many recipients and needs to wait before sending more email. | The email sender has exceeded the maximum number of sent email allowed. |
| CDRBL | The sending IP address has been temporarily blocked by Cox due to exhibiting spam-like behavior. | The block duration varies depending on reputation and other factors, but will not exceed 24 hours. Inspect email traffic for potential spam, and retry email delivery. |
| IPBL0001 | The sending IP address is listed in the Spamhaus Zen DNSBL. | Refer to Spamhaus for more information and removal instructions. Spamhaus has sole discretion whether to remove the sending IP address from the DNSBL. |
| IPBL0010 | The sending IP is listed in the Return Path DNSBL. | Refer to Return Path for more information and removal instructions. Return Path has sole discretion whether to remove the sending IP address from the DNSBL. |
| IPBL0100 | The sending IP is listed in the Invaluement ivmSIP DNSBL. | Refer to ivmSIP for more information and removal instructions. ivmSIP has sole discretion whether to remove the sending IP address from the DNSBL. |
| IPBL0011 | The sending IP is in the Spamhaus Zen and Return Path DNSBLs. | Refer to the instructions above for error codes IPBL0001 and IPBL0010. |
| IPBL0101 | The sending IP is in the Spamhaus Zen and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL0001 and IPBL0100. |
| IPBL0110 | The sending IP is in the Return Path and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL0010 and IPBL0100. |
| IPBL0111 | The sending IP is in the Spamhaus Zen, Return Path and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL0001, IPBL0010, and IPBL0100. |
| IPBL1000 | Cloudmark CSI Suspect / CSI Global | The sending IP address is listed on a CSI blacklist. You can check your status on the CSI website. |
| IPBL1001 | The sending IP is listed in the Cloudmark CSI and Spamhaus Zen DNSBLs. | Refer to the instructions above for error codes IPBL1000 and IPBL0001. |
| IPBL1010 | The sending IP is listed in the Cloudmark CSI and Return Path DNSBLs. | Refer to the instructions above for error codes IPBL1000 and IPBL0010. |
| IPBL1011 | The sending IP is in the Cloudmark CSI, Spamhaus Zen and Return Path DNSBLs. | Refer to the instructions above for error codes IPBL1000, IPBL0001, and IPBL0010. |
| IPBL1100 | The sending IP is listed in the Cloudmark CSI and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL1000 and IPBL0100. |
| IPBL1101 | The sending IP is in the Cloudmark CSI, Spamhaus Zen and Invaluement IVMsip DNSBLs. | Refer to the instructions above for error codes IPBL1000, IPBL0001, and IPBL0100. |
| IPBL1110 | The sending IP is in the Cloudmark CSI, Return Path and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL1000, IPL0010, and IPBL0100. |
| IPBL1111 | The sending IP is in the Cloudmark CSI, Spamhaus Zen, Return Path and Invaluement ivmSIP DNSBLs. | Refer to the instructions above for error codes IPBL1000, IPBL0001, IPBL0010, and IPBL0100. |
| IPBL00001 | Spamhaus ZEN | The sending IP address is listed on a Spamhaus blacklist. Check your status at Spamhaus. |
| URLBL011 | A URL within the body of the message was found on blocklists SURBL and Spamhaus DBL. | Refer to SURBL for more information and removal instructions. Refer to Spamhaus for DBL information and removal instructions |
| URLBL101 | A URL within the body of the message was found on blocklists SURBL and ivmURI. | Refer to SURBL for more information and removal instructions. Refer to ivmURI for more information and removal instructions |
| URLBL110 | A URL within the body of the message was found on blocklists Spamhaus DBL and ivmURI. | Refer to ivmURI for more information and removal instructions. Refer to Spamhaus for DBL information and removal instructions |
| URLBL1001 | Spamhaus DBL | The URL is listed on a Spamhaus blacklist. Check your status at Spamhaus. |
| 421 – [ESMTP server temporarily not available] | Our systems are experiencing an issue which is causing a temporary inability to accept new email. | Retry your mailings at a later time. |
| 421 – [too many sessions from <source IP>] | The sending IP address has exceeded the five maximum concurrent connection limit. | Configure your sending server to establish no more than five concurrent connections. |
| 554 – [ESMTP no data before greeting] | The sending server has attempted to communicate too soon within the SMTP transaction. | Ensure that your sending server is RFC 5321-compliant, and is following the proper SMTP protocol standards. |
| 554 – [<source IP> rejected – no rDNS] | Cox requires that all connecting email servers contain valid reverse DNS PTR records. | Ensure that your sending email servers have a valid reverse DNS entry. |
| 554 – [cox too many bad commands from <source IP>] | An email client has repeatedly sent bad commands or invalid passwords resulting in a three-hour block of the client’s IP address. | Check the password used by your email client and consider using an alternate email client if your password is correct. |
| 421 – [<source IP> DNS check failure – try again later] | The reverse DNS check of the sending server IP address has failed. | Ensure that your DNS configuration is correct and that the PTR record is resolvable. |
| 452 – [<mailfrom address> requested action aborted: try again later] | The SMTP connection has exceeded the 100 email message threshold and was disconnected. | Establish a new connection and continue sending. |
| 550 – [<mailfrom address> sender rejected] | Cox requires that all sender domains resolve to a valid MX or A-record within DNS. | Ensure that your domain has a valid MX or A-record within DNS. |
| 550 – [<recipient email> recipient rejected] | The intended recipient is not a valid Cox Email account. | Ensure that your mailing list is up-to-date with verified recipient email addresses. |
| 552 – [$(_id) attachment extension is forbidden] | The message has been rejected because it contains an attachment with one of the following prohibited file types, which commonly contain viruses: .shb, .shs, .vbe, .vbs, .wsc, .wsf, .wsh, .pif, .msc, .msi, .msp, .reg, .sct, .bat, .chm, .isp, .cpl, .js, .jse, .scr, .exe. | Ensure that your mailings do not contain attachments with these file types. |
| 452 – [<source IP> rejected. Too many invalid recipients] | The sending IP address has exceeded the threshold of invalid recipients and has been blocked. | Ensure that your mailing list is up-to-date with verified recipient email addresses, and try again at a later time. |
| 452 – [Message threshold exceeded] | Cox enforces various rate limits to protect our platform. The sending IP address has exceeded one of these rate limits and has been temporarily blocked. | Slow down the rate at which you attempt to send email. |
COMCAST error codes
| Error Code Returned | For more information |
|---|---|
| 421 – [Too many sessions opened] | Comcast allows 25 simultaneous connections per sending IP address. This error results when that limit is exceeded. |
| 421 – [Reverse DNS failure : Try again later] | Comcast requires all sending IP addresses have a valid rDNS. This error results when the lookup failed. This error should be treated as a temp-fail and can be retried. SERVFAIL response. 99.9% means that the DNS somewhere up the auth tree is misconfigured or down or very badly overloaded. |
| 421 – [Try again later] | This error results for several reasons. This error should be treated as a temp-fail and can be retried. |
| 452 – [Too many emails sent on this session] | Comcast allows 1000 emails per session. This error results when that limit is exceeded. |
| 452 – [Too many recipients for message] | Comcast allows 100 recipients per message. This error results when that limit is exceeded. |
| 550 – [Not our customer] | This error results when a message is sent to a non-existent Comcast customer. These errors should be treated as an “unsubscribe” by the sender, if a bulk mailer. |
| 550 – [Your message could not be delivered due to too many invalid recipients] | This error results when a message is sent to too many invalid recipients. As a result none of the recipients received the message. Please check your distribution list and resend. |
| 550 – [Invalid sender domain] | Comcast requires that all sending domains have a valid A or MX record. This error results when neither record can be found. |
| 550 – [Account not available] | This error results when a message is sent to a Comcast account that is currently not available. |
| 552 – [Message size exceeded] | Comcast allows messages of 15MB or smaller. This error results when that limit is exceeded. |
| 554 – [PTR lookup failure] | Comcast requires all sending mail server IP addresses have a valid PTR record set up. This error results when the lookup failed.
NXDOMAIN response. One of the authoritative servers for the relevant section of the in-addr.arpa DNS tree is saying that there is no PTR record for the given IP address. |
AOL – YAHOO SMTP Error Codes
An SMTP error, or Delivery Status Notification (DSN), indicates that an email could not be delivered, either due to a temporary or permanent problem. You can review our list of SMTP error messages below for details about each error.
What are 4xx (421 and 451) temporary errors?
- A 421 or 451 SMTP error indicates a temporary problem blocking
the delivery of your message. Message delivery can be delayed for
multiple reasons including :
- We’ve observed unusual traffic patterns from your sending server’s IP address.
- The message you attempted to send included characteristics of spam.
- Emails from your mail server have been generating complaints from Verizon Media users.
- The Verizon Media mail servers are busy when attempting a connection.
- Other suspicious behavior which leads Verizon Media to enforce a dynamic deprioritization for your sender’s server.
- Review our Sender Best Practices to learn how you can help your emails reach Verizon Media.
- If you’re not the administrator of the mail server in question, we suggest you contact the administrator directly to discuss the error message you’re receiving.
- If you encounter 4XX errors when trying to send an email to Verizon Media, you may retry sending at a later time.
What are 5xx (553 and 554) permanent errors?
- A 553 or 554 SMTP error indicates an email could not be delivered
due to a permanent problem. Message delivery can be permanently
deferred because :
- Spamhaus has your domain or IP address listed on a Block List.
- You’re trying to send a message to an invalid email address.
- Your message failed authentication checks against your sending domain’s DMARC or DKIM policy.
- The message contains characteristics that Verizon Media won’t accept for policy reasons.
- Other suspicious behavior which leads Verizon Media to issue a permanent rejection for your SMTP connection.
- If you consistently receive 5xx errors when sending to Verizon Media, we encourage you to review our Sender Best Practices, since 5xx errors can be a symptom of a more widespread, general problem.
- You should not retry sending an email that comes back with with a 5xx error. List managers should have a policy for removing email addresses that generate 5xx errors/bounces.
Excessive user complaints
- These errors indicate emails from your mail server are generating
an excessive amount of complaints from Verizon Media users. We suggest
you consider the following :
- Monitor your sender reputation – Even if you have a good reputation, users can vote your email as spam and affect your overall reputation.
- If you’re an administrator of message content and mailing policy and you’ve deployed significant changes or you’ve received this error for more than 48 hours, we ask that you review your outgoing messages for objectionable content or practices.
- Using a shared IP address – Mail traffic from other domains could be negatively affecting your IP sending reputation. If applicable, contact your host provider to request using dedicated IP addresses to send your mail to resolve this problem.
- We recommend you review our Sender Best Practices to ensure you’re following proper opt-in methods of user subscriptions.
Excessive unsolicited messages
- Excessively high volume of emails from a single IP address is a characteristic of unsolicited, bulk emailing. Don’t resend the email until you review our Sender Best Practices and make any necessary changes.
- You may have setup the IP you are using to send email very recently and did not increase the email traffic slowly.
- Check your subscription practices and lists to ensure that messages are sent only to users who’ve requested it.
- If you’re an administrator for message content or mailing policy and you’ve made significant changes, or you’ve received this error for more than 48 hours, we ask that you review your outgoing emails for objectionable content or practices.
Excessive unknown recipients
- Your mail server is sending to a large number of invalid recipients and may be configured as an “open relay” or “open proxy”.
- Open proxies and open relays are a very common source of spam and Verizon Media doesn’t accept email from them.
- We encourage you to follow our Sender Best Practices and make appropriate changes to secure your servers.
- Review your mailing lists and remove any addresses that generate bounces.
- Examine your outbound queues for potentially objectionable content to ensure that spammers aren’t abusing your mail server.
Authentication failures
- Your email failed one or more authentication checks that Verizon Media uses to verify emails are truly sent from the domains they claim to originate from.
- Verizon Media rejects emails for failing DKIM authentication when all of following conditions apply :
- The signing domain publishes a policy which states that all emails from the domain must be signed and authenticated with DKIM to prevent forgery.
- The signing domain is identified in the “d=” tag of the DKIM signature.
- The rejected email couldn’t be authenticated against the sending domain’s policy, for example, due to a missing or bad signature.
- If you’re not the system administrator for the mail servers affected, we encourage you to contact the administrator, so they can look into the situation further.
- For mailing lists, also known as “listservs,” you should change your sending behavior by adding the mailing lists’ address to the “From:” line, rather than the sender’s address. Also, enter the actual user/sender address into the “Reply-To:” line.
- It’s a good idea to review our guide to Sender Best Practices and make appropriate changes.
Content based blocks
- These error messages indicates that your email wasn’t accepted because there is something in the content that Verizon Media won’t accept for policy reasons.
- Objectionable content that Verizon Media deems unacceptable includes :
- Viruses
- Phishing attempts
- Ransomeware
- Other malicious software
- Links or URLs to any of the above
- Examine your outbound queues for potentially objectionable content to ensure that spammers aren’t abusing your mail server.
- Follow our Sender Best Practices and make appropriate changes to secure your servers.
Recipient does not exist
- The Verizon Media account that you’re trying to send to does not exist. We recommend that you contact the recipient directly to confirm their correct email address.
- You should not retry delivery of the message. It will never complete successfully.
- You should remove the email address from your mailing list.
Message temporarily deferred
- This is a temporary error and your mail server may automatically
re-try sending the email at a later time. Your message may have been
deferred due to one or more of the following :
- Emails from your mail server are generating substantial complaints from Verizon Media users.
- The message contained objectionable content or exhibited characteristics indicative of spam.
- The IP (x.x.x.x or its subnet, i.e., .255 ) has a poor reputation.
- We are seeing unusual traffic patterns from your mail servers.
- We ask that you follow our Sender Best Practices and review your outgoing messages for potentially objectionable content.
- If your mail server does not primarily send bulk mailings (e.g., you run a personal, corporate, educational, or ISP mail server), examine your outbound queues to ensure that spammers aren’t abusing your mail server.
- If you’re seeing the same error consistently over an extended period of time, we encourage you to submit a Sender Support Request with specific details of the error and diagnostic codes you see in your logs.
Resources temporarily unavailable
- This error indicates that the Verizon Media mail servers were busy and temporarily unable to process your transaction at the time of connection.
- Such issues are generally brief, and normal connectivity is usually restored after a short period of time.
Sample Email Failed Delivery message
- When you get mail from a “MAILER-DAEMON” or a “Mail Delivery Subsystem” with a subject similar to “Failed Delivery,” this means that a message you sent was undeliverable and has been bounced back to you.
- These messages are produced automatically and usually include a reason for the delivery failure.
- Failed or “bounced” messages normally consist of two parts:
- The reason for the bounce
- Your original message
- Failure notice example:
Define if SOFT or HARD bounces
| Bounce code | Bounce type | Description |
| 5.0.0 | Hard | Address does not exist |
| 5.1.0 | Hard | Other address status |
| 5.1.1 | Hard | Bad destination mailbox address |
| 5.1.2 | Hard | Bad destination system address |
| 5.1.3 | Hard | Bad destination mailbox address syntax |
| 5.1.4 | Hard | Destination mailbox address ambiguous |
| 5.1.5 | Hard | Destination mailbox address valid |
| 5.1.6 | Hard | Mailbox has moved |
| 5.1.7 | Hard | Bad sender\’s mailbox address syntax |
| 5.1.8 | Hard | Bad sender’s system address |
| 5.2.0 | Soft | Other or undefined mailbox status |
| 5.2.1 | Soft | Mailbox disabled, not accepting messages |
| 5.2.2 | Soft | Mailbox full |
| 5.2.3 | Hard | Message length exceeds administrative limit. |
| 5.2.4 | Hard | Mailing list expansion problem |
| 5.3.0 | Hard | Other or undefined mail system status |
| 5.3.1 | Soft | Mail system full |
| 5.3.2 | Hard | System not accepting network messages |
| 5.3.3 | Hard | System not capable of selected features |
| 5.3.4 | Hard | Message too big for system |
| 5.4.0 | Hard | Other or undefined network or routing status |
| 5.4.1 | Hard | No answer from host |
| 5.4.2 | Hard | Bad connection |
| 5.4.3 | Hard | Routing server failure |
| 5.4.4 | Hard | Unable to route |
| 5.4.5 | Soft | Network congestion |
| 5.4.6 | Hard | Routing loop detected |
| 5.4.7 | Hard | Delivery time expired |
| 5.5.0 | Hard | Other or undefined protocol status |
| 5.5.1 | Hard | Invalid command |
| 5.5.2 | Hard | Syntax error |
| 5.5.3 | Soft | Too many recipients |
| 5.5.4 | Hard | Invalid command arguments |
| 5.5.5 | Hard | Wrong protocol version |
| 5.6.0 | Hard | Other or undefined media error |
| 5.6.1 | Hard | Media not supported |
| 5.6.2 | Hard | Conversion required and prohibited |
| 5.6.3 | Hard | Conversion required but not supported |
| 5.6.4 | Hard | Conversion with loss performed |
| 5.6.5 | Hard | Conversion failed |
| 5.7.0 | Hard | Other or undefined security status |
| 5.7.1 | Hard | Delivery not authorized, message refused |
| 5.7.2 | Hard | Mailing list expansion prohibited |
| 5.7.3 | Hard | Security conversion required but not possible |
| 5.7.4 | Hard | Security features not supported |
| 5.7.5 | Hard | Cryptographic failure |
| 5.7.6 | Hard | Cryptographic algorithm not supported |
| 5.7.7 | Hard | Message integrity failure |
| 9.1.1 | Hard | Hard bounce with no bounce code found. It could be an invalid email or rejected email from your mail server (such as from a sending limit). |
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 fulfil 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 |
Exchange NDR Code List
Listed NDR (Nondeliverable returns) emails and codes. Overlapping with the codes above.
| Status code | Description |
| 4.2.2 | The recipient has exceeded their mailbox limit |
| 4.3.1 | Insufficient system resources |
| 4.3.2 | System not accepting network messages |
| 4.4.1 | Connection timed out |
| 4.4.2 | Connection dropped |
| 4.4.6 | Too many hops |
| 4.4.7 | Message expired |
| 4.4.9 | DNS Problem – check your connector |
| 4.6.5 | Multi-language situation |
| 5.0.0 | HELO / EHLO requires domain address |
| 5.1.x | Exchange NDR problems with email address |
| 5.1.0 | Sender denied / Message categorizer failures |
| 5.1.1 | Bad destination mailbox address |
| 5.1.2 | Invalid X.400 address |
| 5.1.3 | Invalid recipient address |
| 5.1.4 | Destination mailbox address ambiguous |
| 5.1.7 | Invalid address |
| 5.1.8 | Something the matter with sender’s address |
| 5.2.x | NDR caused by the size of the email |
| 5.2.1 | Mailbox cannot be accessed |
| 5.2.2 | Mailbox full |
| 5.2.3 | Message too large |
| 5.2.4 | Mailing list expansion problem |
| 5.3.0 | Problem with MTA / Store driver |
| 5.3.1 | Mail system full |
| 5.3.2 | System not accepting network messages |
| 5.3.3 | Unrecognized command |
| 5.3.4 | Message too big for system |
| 5.3.5 | System incorrectly configured |
| 5.4.0 | DNS Problem. Check the Smart host and DNS |
| 5.4.1 | No answer from host |
| 5.4.2 | Bad connection |
| 5.4.3 | Routing server failure. No available route |
| 5.4.4 | Invalid arguments |
| 5.4.6 | Routing loop detected |
| 5.4.7 | Delivery time-out. Message is taking too long to be delivered |
| 5.4.8 | Microsoft advise, check your recipient policy |
| 5.5.0 | Underlying SMTP 500 error |
| 5.5.1 | Invalid command |
| 5.5.2 | Send hello first |
| 5.5.3 | Too many recipients |
| 5.5.4 | Invalid domain name |
| 5.5.5 | Wrong protocol version |
| 5.5.6 | Invalid message content |
| 5.6.0 | Corrupt message content. Try sending without attachment |
| 5.6.1 | Media not supported |
| 5.6.3 | More than 250 attachments |
| 5.7.1 | Delivery not authorized |
| 5.7.1 | Unable to relay |
| 5.7.1 | Client was not authenticated |
| 5.7.2 | Distribution list cannot expand and so is unable to deliver its messages |
| 5.7.3 | Not Authorized |
| 5.7.4 | Extra security features not supported. Check delivery server settings |
| 5.7.5 | Cryptographic failure |
| 5.7.6 | Certificate problem, encryption level may be to high |
| 5.7.7 | Message integrity problem |