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  • This site provides general information regarding email delivery to and Cox Business customers.

Welcome to the Postmaster Site
This site provides general information regarding email delivery to and Cox Business customers. It is intended as a resource for email administrators to understand what they can do to improve their deliverability rate as well as what to do when they encounter problems. Please review this information and follow the instructions prior to contacting Cox regarding any deliverability issues.

General Guidelines for Successful Delivery
Operating within these guidelines will improve delivery of your email.

  • Monitor RBL listings for your IPs and domains. Cox uses several IP and domain-based RBL's (real-time blacklists) to make filtering decisions. Ensuring that your IPs and domains are not listed on these types of lists will reduce your chances of being blocked by Cox as well as other providers.
  • Maintain DNS records for your IPs and domains. Cox requires that all sending IP addresses resolve to a valid reverse DNS entry. All sending domains must resolve to valid A or MX record. Connections from IPs or domains that do not have valid DNS entries will be rejected.
  • Ensure mailing lists contain valid recipients. Make sure that mailings are sent to valid recipients. Mail sent to a large number of invalid or non-existent recipients can result in the IP or domain being blocked.
  • Monitor your IP and domain reputation. Cox tracks the reputation of sending IPs and domains using data from spam complaints and spamtraps, as well as external reputation data sources. Maintaining a clean reputation will help minimize deliverability issues.
  • Investigate SMTP Rejection Messages. When a sender encounters either a temporary (4xx) or permanent (5xx) rejection, the error message will contain a code and information about the reason for the block. Please see the Error Codes section for more information about the possible errors that can be encountered.

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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