I am a system administrator for a small ISP that is having the same problem as some others have described. Delivery from our mail server to Comcast email addresses is showing up as:
to=<firstname.lastname@example.org>, relay=mx2.comcast.net[184.108.40.206]:25, delay=1410, delays=1393/0/16/0.29, dsn=4.1.0, status=deferred (host mx2.comcast.net[220.127.116.11] said: 421 4.1.0 18.104.22.168 Throttled - try again later. Please see http://postmaster.comcast.net/smtp-error-codes.php#RL000003
A ticket NA250188731 has been submitted since 7/22/2019 with no resolution yet. Our mail server is on no blacklists and we have long standing zero tolerance for any abuse on our mail system. Is there any hope of getting this problem addressed soon?
UPDATE: A workaround for us was to apply a transport limit rate delivery to our mail servers. This will slow down the delivery speed specifically and only to the Comcast mail servers from our mail servers, which reduces the throttling error that the Comcast mail servers throw up. The downside is a temporary queue build up as the mail is slowly delivered to Comcast email addresses.
Greetings, rgnfn! I appreciate you taking the time to reach out to us here on the forums. I hope you are having an amazing day. I apologize to hear you are running into this issue with the mail server. That's certainly frustrating. I do believe with that type of ticket number you would need to reach out to our CSA department. They can be reached at 1-888-565-4329 and should be able to provide with an update. I apologize again for the inconvenience.
Hello, rgnfn. I have not heard back from you. If you still need assistance, please feel free to start a new public thread.