Comcast Delivery Delays

We’ve heard from end mailers and from top-tier email service providers who are experiencing large queues attempting to deliver email to Comcast, one of the leading cable providers in North America. More than 5 separate email service providers have reported queue times from hours to days. The behavior appears to have started on Friday, May 14th and has persisted till today, Friday the 21st of May. Comcast is looking into the issue and working with one ESP to help them, Comcast,  trouble shoot the problem as they are not seeing any specific problems that may be leading to the delays on their end.


Marketers can expect to see little to no clicks or reads on their emails bound for Comcast domains until the issue is resolved. The delivery delays vary from day to day so there’s no real benchmark on how long it will take for mail to arrive until the issue is resolved. Marketers should consider pausing their time sensitive offers and or extending the time periods for Comcast users as they could potentially not see the offer until the time frame expires.

The specific error mailers are seeing has varied a bit from a 421 Throttling Error to a 451 Banner issue. In both cases the error code is consistent with a transient failure so mailers should continue to retry but be sure to pay special attention to the extended error code as it could be a completely different issue. A complete list of Comcast’s error codes can be found on their post master site here. The latest we’ve heard is that Comcast believes this could be a resolution issue and is investigating it.

We will continue to monitor the issue and update this post accordingly.

-Len Shneyder
Director of Deliverability & Messaging
Unica | Pivotal Veracity


  • Len Shneyder says:

    Comcast personnel infomred Unica’s Email Operations team this morning that the problem has been resolved. The information that we provided to their postmaster team was very helpful in pin pointing the delivery delay issue and resolving it. Marketers should be able to deliver messages without long delivery delays to any Comcast properties. If you’re still having issues delivering messages we recommend visiting

    Happy Monday!

  • Len Shneyder says:

    We received news today that the delivery delays to Comcast have returned. We’ve touched base with Comcast personnel and learned that they are investigating the problem and hoping to have a resolution shortly. I’ll update this post once they’ve confirmed resolution or with further details about the problem.


  • Tom says:

    As of June 8, 2010, are the email delivery delays with Comcast still occurring? Since Friday, June 4, we have been receiving messages indicating that email delivery to Comcast accounts have been delayed.

Leave a Reply

Your email address will not be published. Required fields are marked *