Cox provides high speed network access as well as mailboxes for their subscribers.

Delivering to Cox

Cox throttles mail based on sender reputation, both IP based and domain based. Senders are limited to opening 5 connections per IP address . They also have a published limit of 100 messages per connection. In addition to those limits, Cox will throttle based on sender reputation.
Cox requires valid reverse DNS on all sending IPs. All sending domains must have a valid A or MX record.
Some business domains are hosted by Cox, but they do not offer general hosting. Email addresses in the domain belong to subscribers, addresses in the domain are reserved for Cox employees. This page discusses filtering and limits on the email addresses.

Important links and references

Postmaster pages

Cox has an extensive set of postmaster pages providing information about delivering mail to email addresses.

Feedback loop

There is a feedback loop for email addresses. The FBL is administered by Return Path. Applications are accepted at the Cox FBL signup page. IP addresses will only be accepted if their SenderScore is above 60 or if their SenderScore is between 30 and 60 and the IP does not appear on any DNSBLs. IP addresses with a SenderScore below 30 will not be accepted.


Cox does not offer whitelisting services.

Certification Services

Cox uses the Return Path Certified whitelist in mail delivery decisions. Mail from IPs on the Return Path Certified list will see better delivery at Cox domains.

Support for Senders

As part of their postmaster webpages, they maintain a comprehensive list of error codes returned in response to blocked email. They also provide a blocklist removal form. Cox does not provide any formal way to contact them directly.

Known spamfilters

Cox uses a number of commercial spam filters and spamfiltering services. For IP filtering Cox is using Spamhaus Zen, the Return Path DNSBL, the Invaluement SIP list, and Cloudmark CSI. Cox is also using multiple lists for domain based filtering, including SURBL, Spamhaus DBL, and the Invaluement URI list.


Updated 3/13/2014

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