Sending an email from Google to BT Internet and the message was rejected with the following message:
Google tried to deliver your message, but it was rejected by the server for the recipient domain <a href="http://btinternet.com" target="_blank">btinternet.com</a> by <a href="http://mx.bt.lon5.cpcloud.co.uk" target="_blank">mx.bt.lon5.cpcloud.co.uk</a>. [184.108.40.206]. The error that the other server returned was: 421 Policy (220.127.116.11) Too much SPAM received from 18.104.22.168 on 2019/02/01 11:41:07 GMT, if you share a mail server consider moving to a dedicated mail server not sending SPAM. Guide for senders <a href="http://www.bt.com/postmaster" target="_blank">www.bt.com/postmaster</a>
Surely Google's MX's should be Whitelisted given the number of users who use Google's mail services?