A Marketer’s Field Guide to Comcast Inboxes

Comcast is the largest cable provider in the United States with 22 million subscribers comprising 32 million active mailboxes. Their email user base is made up of paid subscribers to their Internet service. Comcast provides a reliable email service with lots of storage. Unlike other North American cable providers, Comcast has invested significantly in their email service which includes a snappy Zimbra-based web interface.

Comcast has a dedicated postmaster and abuse team. Their Postmaster site is robust and the email best practices page is a good reference for any sender. Their team has implemented a reliable ticketing system with automated responses. We find that our client issues are resolved easily through this system and we rarely see long-term deliverability problems with good senders.

Spam filtering. Comcast has developed their own proprietary filtering system but their Cloudmark filters make up a large part of their decisions.

If you are not familiar with Cloudmark, you should be. Cloudmark operates a global threat network that is leveraged by many global ISPs. If your email has been marked as spam by Cloudmark you will experience issues not only at Comcast but at many other receivers that have implemented their filters and subscribe to their network. Cloudmark looks at both IP reputation and content to make filtering decisions. Cloudmark filters and flags content as spam or suspected spam when they receive complaints from ISPs or when users of Cloudmark software vote the email as spam.

Blocking is a common issue we see with Comcast and is generally the result of spam complaints. Comcast subscribes to several blacklists including Spamhaus, TrendMicro MAPS and Return Path. If you are seeing blocking at Comcast’s gateway you should cross reference your SMTP logs with Comcast’s Mail Delivery Error Codes list.

Good list hygiene is also an important factor in inbox placement. Unknown user rates greater than 10% and 1 spam trap per million email addresses may reduce your inbox placement rate.

Feedback loops. Comcast offers users a “This is Spam” reporting mechanism in their web email interface. Signing up for Comcast’s feedback loops will allow you to remove these complainers from your list. Most senders see considerable delivery improvements after signing up for the Comcast feedback loop and removing users who complain.

Prioritized Delivery. Comcast subscribes to Return Path’s Certified Whitelist which allows sending IPs preferential treatment by Comcast’s spam filters to deliver the majority of their email to the inbox with images on and links enabled.

Sending reputation. Comcast subscribes to Return Path’s Sender Score reputation service and will make throttling decisions based on your score. Throttle rates are based on authentication as well as sender reputation. They recently tightened their connection and throughput settings so check your MTA configuration to make sure it conforms to the new policy of 25 simultaneous connections with up to 100 recipients per message.

Engagement. Comcast does not have user level engagement filters at this time. Neither does their webmail interface allow individual whitelisting and blacklisting. All deliverability decisions are made on a global basis.

Sending Infrastructure requirements. It is a best practice to sign all email with SPF and DKIM as many ISPs will bulk you if you fail both but failing these checks won’t affect inbox placement at Comcast.

That’s it for Comcast. Stay tuned for the next blog in my Email Marketer’s Field Guide series! Have you missed any?

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