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Error Code Details

The following is a list of the possible error codes that a sender may encounter. A brief description is provided along with instructions on how to address the issue.

Error Code Description What To Do
CXBL The sending IP address has been blocked by Cox due to exhibiting spam-like behavior Submit a request using the contact form here. The request will be reviewed by Cox. Cox has sole discretion whether to unblock the sending IP address.
CDRBL The sending IP address has been temporarily blocked by Cox due to exhibiting behavior characteristic of spam The duration of the block may vary depending on reputation and other factors, but will not exceed 24 hours. Inspect message traffic for potential spam, and retry message delivery.
IPBL0001 The sending IP is listed in the Spamhaus Zen DNSBL. Click here for more information and removal instructions from Spamhaus. Spamhaus has sole discretion whether to remove the sending IP from the DNSBL.
IPBL0010 The sending IP is listed in the Return Path DNSBL Click here for more information and removal instructions from Return Path. Return Path has sole discretion whether to remove the sending IP from the DNSBL.
IPBL0100 The sending IP is listed in the Invaluement ivmSIP DNSBL Click here for more information and removal instructions from ivmSIP. ivmSIP has sole discretion whether to remove the sending IP from the DNSBL.
IPBL0011 The sending IP is in the Spamhaus Zen and Return Path DNSBLs See the "What To Do" instructions above for Spamhaus and Return Path
IPBL0101 The sending IP is in the Spamhaus Zen and Invaluement ivmSIP DNSBLs See the "What To Do" instructions above for Spamhaus and ivmSIP
IPBL0110 The sending IP is in the Return Path and Invaluement ivmSIP DNSBLs See the "What To Do" instructions above for Return Path and ivmSIP
IPBL0111 The sending IP is in the Spamhaus Zen, Return Path and Invaluement ivmSIP DNSBLs See the "What To Do" instructions above for Spamhaus Zen, Return Path and ivmSIP
IPBL1000 The Sending IP is listed in the Cloudmark CSI DNSBL Click here for more information and removal instructions from Cloudmark CSI. Cloudmark has sole discretion whether to remove the sending IP from the DNSBL.
IPBL1001 The sending IP is listed in the Cloudmark CSI and Spamhaus Zen DNSBLs. See the "What To Do" instructions above for Cloudmark CSI and Spamhaus Zen.
IPBL1010 The sending IP is listed in the Cloudmark CSI and Return Path DNSBLs. See the "What To Do" instructions above for Cloudmark CSI and Return Path.
IPBL1011 The sending IP is in the Cloudmark CSI, Spamhaus Zen and Return Path DNSBLs. See the "What To Do" instructions above for Cloudmark CSI, Spamhaus and Return Path.
IPBL1100 The sending IP is listed in the Cloudmark CSI and Invaluement ivmSIP DNSBLs. See the "What To Do" instructions above for Cloudmark CSI and Invaluement ivmSIP.
IPBL1101 The sending IP is in the Cloudmark CSI, Spamhaus Zen and Invaluement IVMsip DNSBLs. See the "What To Do" instructions above for Cloudmark CSI, Spamhaus and ivmSIP.
IPBL1110 The sending IP is in the Cloudmark CSI, Return Path and Invaluement ivmSIP DNSBLs. See the "What To Do" instructions above for Cloudmark CSI, Return Path and ivmSIP.
IPBL1111 The sending IP is in the Cloudmark CSI, Spamhaus Zen, Return Path and Invaluement ivmSIP DNSBLs. See the "What To Do" instructions above for Cloudmark CSI, Spamhaus Zen, Return Path and ivmSIP.
URLBL011 A URL within the body of the message was found on blocklists SURBL and Spamhaus DBL Click here for SURBL information and removal instructions. Click here for Spamhaus DBL information and removal instructions
URLBL101 A URL within the body of the message was found on blocklists SURBL and ivmURI Click here for SURBL information and removal instructions. Click here for ivmURI information and removal instructions
URLBL110 A URL within the body of the message was found on blocklists Spamhaus DBL and ivmURI Click here for ivmURI information and removal instructions. Click here for Spamhaus DBL information and removal instructions
421 - [ESMTP server temporarily not available] Our systems are experiencing an issue which is causing a temporary inability to accept new messages Retry your mailings at a later time
421 - [too many sessions from <source IP>] The sending IP has exceeded the maximum concurrent connection limit, currently 5 Configure your sending server to establish no more than 5 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 mail servers contain valid reverse DNS (PTR) records Ensure that your sending mail servers have a valid reverse DNS entry
554 - [cox too many bad commands from <source IP>] A mail 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 mail client and consider using an alternate mail 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 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 DNS record 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.net account Ensure that your mailing list is up-to-date with verified recipient 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 has exceeded the threshold of invalid recipients and has been blocked Ensure that your mailing list is up-to-date with verified recipient addresses, and try again at a later time
452 - [Message threshold exceeded] Cox enforces various rate-limits to protect our platform. The sending IP has exceeded one of these rate-limits and has been temporarily blocked Slow down the rate at which you attempt to send messages.

Why am I being blocked?

The error message that was displayed at the time of the block contains information regarding the reason. . More details about the various error messages can be found in the Error Codes section.

Can I get unblocked?

This depends on the reason for the block. The details of the error you encountered are found in the Error Codes section, which contains information on how to request a block be lifted.

Can I be whitelisted?

Protection of our customers and network is paramount. Therefore, we do not whitelist.

Does Cox offer a Feedback Loop?

Yes. See the Feedback Loop section here.

What are Cox's sending limits?

Cox currently allows up to 5 simultaneous connections per sending IP address, and up to 100 messages per connection. However, regardless of these limits, Cox may throttle your sending ability based on your sender reputation. Cox's sending limits and policies are subject to change at any time. Cox will update this website when changes are made.

How do I report abuse?

Please find instructions on how to report abuse here.


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