Comcast

Comcast provides high speed network access as well as mailboxes for their subscribers. They have over 14 million subscribers¹ and each subscriber line has access to multiple email addresses.

Delivering to Comcast

Comcast throttles mail based on sender reputation, both IP based and domain based. Senders are limited to opening 25 connections per IP address. They also have a published limit of 100 recipients per message.
Some business domains are hosted by Comcast, but Comcast does not offer general hosting. Email addresses in the comcast.net domain belong to subscribers, addresses in the comcast.com domain are reserved for Comcast employees. This page discusses filtering and limits on the comcast.net email addresses.

Important links and references

Postmaster pages

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

Feedback loop

There is a feedback loop for comcast.net email addresses. The FBL is administered by Return Path. Applications are accepted at the Comcast FBL signup page. Comcast sends reports in ARF format, but does scrub the recipient’s email address before sending.

Whitelist

Comcast does not offer whitelisting services.

Authentication

Comcast does reject mail based on DMARC records.

Certification Services

Comcast uses the Return Path Certified whitelist in mail delivery decisions. Mail from IPs on the Return Path Certified list will see better delivery at Comcast 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. Comcast does not provide any formal way to contact them directly.

Known spamfilters

Comcast uses a number of commercial spam filters and spamfiltering services. These include the Spamhaus Spamhaus ZEN, Cloudmark CSI, and Return Path Reputation Network Blacklist. 

Notes

Updated 3/12/14