Hello, I have seen some reports of this issue on these forums. We are a local government and sending to a mailing list. Here is an example message trace we are getting for all Comcast addresses in our mailing list. We keep seeing these every couple minutes until the email eventually times out.
Message # to XXXX@comcast.net delayed. Reason: 4.3.2 - Not accepting messages at this time ('421', ['4.2.0 IPADDRESS Throttled - try again later. Please see http://postmaster.comcast.net/smtp-error-codes.php#RL000003']) 
Solved! Go to Solution.
@irowell See it that sorts it out for you. I gave that IP a temporary override to help train the system.