Update: The service disruption started at about 1:00 pm EST and resolved at about 2:00 pm EST.
The practical impact of this in most cases was that if a Comcast subscriber attempted to access web interface using HTTPS that attempt was likely to fail.
As far as we can tell, this issue did not impact any unencrypted protocols like HTTP or SMTP, so most outgoing emails and incoming clicks/opens were unaffected.
The problem was resolved on its own.
Comcast users may have intermittent difficulty in accessing the Inboxfirst web interface due to a localized Comcast outage near the data center.