OK - here it is -
Your message wasn't delivered. Despite repeated attempts to deliver your message, the recipient's email system refused to accept a connection from your email system.
Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system is refusing connections from your email server. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.
For Email Admins
No connection could be made because the target computer actively refused it. This usually results from trying to connect to a service that is inactive on the remote host - that is, one with no server application running. For more information and tips to fix this issue see this article:
Fix email delivery issues for error code 550 4.4.7 in Exchange Online
Generating server: SN1NAM04HT109.mail.protection.outlook.com
Receiving server: SN1NAM04HT109.eop-NAM04.prod.protection.outlook.com
marianne.whalen@bcbsri.com
10/30/2019 6:08:12 PM - Server at SN1NAM04HT109.eop-NAM04.prod.protection.outlook.com returned '550 5.4.316 Message expired, connection refused(Socket error code 10061)'
10/30/2019 6:05:50 PM - Server at bcbsri.com (204.146.168.193) returned '450 4.4.316 Connection refused [Message=Socket error code 10061] [LastAttemptedServerName=bcbsri.com] [LastAttemptedIP=204.146.168.193:25] [CO1NAM04FT064.eop-NAM04.prod.protection.outlook.com](Socket error code 10061)'
It's Blue Cross so maybe they have aggressive anti-spam measures in place. I'll just give them a call and inform of problem. Thanks.