I found this page a year ago and since then it has disappeared; it’s not cached on Google and Archive.org didn’t have it either. Luckily I found a post that had re-published the information (instead of just providing a link like 99% of the sources did). To ensure I have a copy and to help other people out I’m posting this information here.
Why is this useful?
This allows you to peak behind the curtain to avoid being throttled by Comcast (and other ISPs) by sending under their proposed limit, based off your SenderScore. Check your SenderScore
If you are getting a lot of 4XX errors (timed out/try again messages) in your server logs you’re likely being throttled.
If you are starting to send to a huge list (or you haven’t sent a message in the last 90 days) you’ll need to warm up the ISPs using this schedule:
- Send to 10,000 recipients on days 1, 2 and 3
- Send to 20,000 recipients on days 4, 5 & 6
- Send to 40,000 recipients on days 7, 8 & 9
- Send to 80,000 recipients on days 10, 11 & 12
- Send to 160,000 recipients on days 13, 14 & 15
- Send to 300,000 recipients on days 16 +
NOTE: Try to send your 10,000 in small batches each hour. Don’t send all 10,000 as fast as you can. From the list below you’ll get an idea on your sending limit per hour.
Mail to Comcast is queued and I see RL001 in my mail logs. What does this mean?
Comcast has instituted rate limiting on all inbound mail traffic. This step was taken to protect our customers and our mail 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*
3,600 0 – 15
7,200 16 – 25
14,400 26 – 30
28,800 31 – 50
50,400 51 – 70
72,000 71 – 85
86,400 86 – 100
*Subject to successful authentication
I hope this is helpful for those of you that aren’t using an ESP like mailchimp (or other services). Good Luck!