Comcast

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

Delivering to
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 1000 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.

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. 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.

Whitelist
Comcast does not offer whitelisting services.

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 ZEN, Cloudmark CSI , and Return Path Reputation Network Blacklist