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 [126.96.36.199]
Diagnostic-Code: smtp; 550 5.7.0 Blocked - see https://support.proofpoint.com/dnsbl-lookup.cgi?ip=188.8.131.52
Following the link above, message from Proofpoint website:
Dynamic Reputation IP Lookup
This IP address is currently blocked.
IP Address: 184.108.40.206
Query Time: 2020-05-14 09:02:52 UTC
Fixed! Go to the fix.