mb1984's profile

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Thu, Jun 4, 2020 5:00 AM

Comcast Throttling my mail server. New IP, postfix settings well below Comcast sending limits

Hello,  we recently added a new mail server IP

I've checked the postfix logs and it appears every email going to a @comcast.net or @comcast.com email address is throttled  said: 421 4.1.0 Throttled - try again later. Please see http://postmaster.comcast.net/smtp-error-codes.php#RL000001 (in reply to MAIL FROM command)


I am having no issues with any other email domains. 

I have a custom transport rule that only allows our emails to be sent to comcast addresses under the guidelines comcast has set (1 recipient per email - 20 emails within a certain amount of time, etc).


However, once I turned the transport rule on, I have literally 3 emails go out to comcast addresses before we started getting throttled.

Its a new IP so I assume that is the issue,.


Could someone please unflag us, or whitelist our IP for sending to comcast addresses?

This server strictly sends reservation confirmations.  Our customers are waiting for these confirmations


Thank you



Official Employee


580 Messages

1 y ago

  • RL000001 :: Mail to Comcast is queued and I see RL000001 in my email logs. What does this mean?

    Comcast has instituted rate limiting on all inbound email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any systems affected by this rate limit will receive a 4xx message (temp-fail) during the SMTP transaction. This message is designed to instruct the sending server to try again at a later time to deliver its email.

    Our rate limiting policy is based on sender reputation, provided by SenderScore as well as authentication of the sending server, regardless of the number of domains from that server.

    Recipients per Hour SenderScore*
    120 N/A
    1,200 0 - 15
    3,600 16 - 25
    6,000 26 - 30
    12,000 31 - 50
    24,000 51 - 70
    42,000 71 - 85
    60,000 86 - 100
    *Subject to successful authentication

    If the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue.


as the notification suggests, have you tried getting a SenderScore report for your IP and have you tried shortening your servers' delivery retry interval?

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