Comcast troubleshooting support information

Sending successfully to Comcast and reaching the inbox is highly dependent on your sending reputation and following Comcast’s sending best practice guidelines.

Troubleshooting tips

  • Determine if your content contains a spammy Cloudmark fingerprint: Comcast uses Cloudmark in their filtering decisions. After identifying and fixing the cause for the spammy fingerprint, submit a reset request. If you don’t fix the cause of the issue, your content is likely to be marked with a spammy fingerprint in the future.
  • Determine if all sending IP addresses are on Comcast’s complaint feedback loop. One of the common causes for deliverability problems at Comcast are high complaint rates and a low number of people marking your email as “not spam”. Low complaint rates and having subscribers marking your email as “not spam” signals to Comcast that you are less likely a spammer.
  • Determine if your IP address is present on the blacklists used by Comcast.
  • Check your SMTP logs for clues regarding the issue

Comcast blocks email with its own real-time block list. If your email is being throttled or blocked, check your SMTP logs for a response indicating a possible cause.

  • Review your list acquisition and  list hygiene processes: Spam traps are an important reputation measure for Comcast. When they add a new spam trap, they review it to ensure the address is not active in their system. Comcast bounces inactive addresses for six months before the spam trap is created. All Comcast spam traps are at least three to five years old, so Comcast expects senders to not send to them.
  • Determine if your email authentication is working properly: A misconfigured SPF or DKIM record can result in email being filtered or blocked.
  • Check your connection and throughput settings and SMTP error logs: Comcast will throttle your email if you are sending too much email to their servers. Reduce the connection and throughput settings if throttling occurs.
  • Check your retry settings: Increase the time between retries if still experiencing throttling issues. You may want to limit the number of retries per message to 2 or 3 temporarily if needed until you can isolate a cause of the deliverability problems.


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