Throttled email error code 451 4.2.0

Hello, @user_xo8tid this error code looks close to two that are common. 

452 - [Too many emails sent on this session] Comcast allows 1000 emails per session. This error results when that limit is exceeded.

452 - [Too many recipients for message] Comcast allows 100 recipients per message. This error results when that limit is exceeded.

RL000001 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. If the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue. If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329

I should have mentioned the error was for inbound messages from gmail, from a client no bulk sending just one off emails

451 4.2.0 Throttled - https://postmaster.comcast.net/smtp-error-codes.php#RL000010

Hi there, @user_xo8tid A 451 4.2.0 Throttled error from Gmail indicates a temporary deferral due to a rate limit or "suspicious" behavior, often caused by the receiving server (like Comcast/Xfinity) or Google's own filters flagging high-volume, rapid, or unusual traffic, even if not bulk. The message is usually queued and will retry later.-Richard