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.
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.
Comcast has an extensive set of postmaster pages providing information about delivering mail to comcast.net email addresses.
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.
Comcast does not offer whitelisting services.
Comcast does reject mail based on DMARC records.
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.
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.
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.