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 |
N/A & 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
If the rate-limit is impacting your ability to deliver mail, shortening your server's delivery retry interval should resolve the issue.
If you require additional assistance, please contact Customer Security Assurance.