Has anyone got a cure for this -- Neil O had some ideas a few weeks ago
inbound from valid company and valid sender to self @ BT
smtpb1.bt.com gave this error:550 5.7.1 Requested action not taken: message refused
Only a guess but that sounds like an SSL problem to me., either the certificate is not valid or the BT servers don't recognise it.
This may be helpful Fix email delivery issues for error code 550 5.7.1 in Exchange Online | Microsoft Docs