btinternet.com emails rejected by iCloud server

It's seems as though from the beginning of December 2020 my btinternet.com reply emails have been rejected by people writing to me with an iCloud or .me address ..... this is the message I receive .....

Your message was rejected by mx02.mail.icloud.com for the following reason:

    5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137

Any help much appreciated especially with respect to the significance of Dec 1st ? Unless that's a red herring !