Sending email using Plusnet's email servers to an iCloud email account resulted in a bounce due to being blocked by Proofpoint:
Message in bounced email:
Reporting-MTA: dns; avasout01 [220.127.116.11]
Diagnostic-Code: smtp; 550 5.7.0 Blocked - see https://support.proofpoint.com/dnsbl-lookup.cgi?ip=18.104.22.168
Following the link above, message from Proofpoint website:
Dynamic Reputation IP Lookup
This IP address is currently blocked.
IP Address: 22.214.171.124
Query Time: 2020-05-14 09:02:52 UTC
Fixed! Go to the fix.